Issue: [FEATURE] Implement V2 UCS (Unified Connector Service) Integration for Authorization Flow 

### Feature Description

# [Feature Request] Implement V2 UCS (Unified Connector Service) Integration for Authorization Flow

## Issue Type
- [x] Feature Request
- [ ] Bug Report
- [ ] Enhancement
- [ ] Documentation

## Priority
- [x] High
- [ ] Medium 
- [ ] Low

## Labels
`A-core` `C-feature` `payment-flows` `ucs-integration` `v2-api`

## Summary

Implement V2 UCS (Unified Connector Service) integration for the authorization payment flow, upgrading from the existing V1 UCS integration to support the updated gRPC schema and enhanced V2 payment architecture.

## Background

Currently, Hyperswitch has V1 UCS integration that works with the legacy gRPC schema. However, the UCS service has evolved to V2 with improved:
- Enhanced gRPC schema with better type safety
- Improved field structures for card details, addresses, and identifiers
- Better error handling and response parsing
- Enhanced payment method data transformation

## Problem Statement

### Current State
- V1 UCS integration uses outdated `PaymentsAuthorizeRequest` schema
- Limited support for V2 payment flow architecture
- Outdated transformer implementations for V2 data structures
- Monolithic UCS function structure affecting maintainability

### Pain Points
1. **Schema Incompatibility**: V1 gRPC schema doesn't support V2 enhanced data models
2. **Limited Error Handling**: V1 response parsing lacks V2 identifier structure support
3. **Technical Debt**: Monolithic UCS functions make debugging and enhancement difficult
4. **Missing V2 Features**: Cannot leverage V2 payment flow improvements through UCS


### Possible Implementation

## Proposed Solution

### High-Level Approach
Implement V2 UCS integration that:
1. **Updates gRPC Schema**: Migrate to `PaymentServiceAuthorizeRequest` with V2 field structure
2. **Enhances Architecture**: Refactor payment flow with better separation of concerns
3. **Improves Transformers**: Update V2 transformers for new schema compatibility
4. **Maintains Backward Compatibility**: Preserve V1 integration with feature flag separation

### Technical Requirements

#### 1. gRPC Schema Migration
- [ ] Update `rust-grpc-client` dependency to latest version
- [ ] Migrate from `PaymentsAuthorizeRequest` to `PaymentServiceAuthorizeRequest`
- [ ] Update authentication headers and request metadata handling
- [ ] Implement new identifier structure for response parsing

#### 2. Payment Flow Architecture
- [ ] Refactor `call_connector_service()` into separate prerequisite and decision functions
- [ ] Implement `call_connector_service_prerequisites()` for V2 flow preparation
- [ ] Create `decide_unified_connector_service_call()` for UCS routing logic
- [ ] Maintain feature flag separation between V1 and V2 implementations

#### 3. V2 Transformer Enhancements
- [ ] Update card data transformation for new schema structure
- [ ] Enhance address handling with improved field mapping
- [ ] Implement customer ID support in V2 transformers

#### 4. Error Handling & Response Parsing
- [ ] Update response handlers for new identifier structure
- [ ] Enhance error context and fallback mechanisms
- [ ] Maintain automatic fallback to traditional connector flow

#### 5. Testing & Validation
- [ ] Create comprehensive test cases for V2 UCS flow
- [ ] Test schema compatibility and field mapping
- [ ] Validate fallback mechanisms and error handling



### Have you spent some time checking if this feature request has been raised before?

- [x] I checked and didn't find a similar issue

### Have you read the Contributing Guidelines?

- [x] I have read the [Contributing Guidelines](https://github.com/juspay/hyperswitch/blob/main/docs/CONTRIBUTING.md)

### Are you willing to submit a PR?

Yes, I am willing to submit a PR!

Resolution: <think>
PR Context:
- Title: feat(core): Hyperswitch <|> UCS integration v2
- Merged: 2025-07-11T14:44:46Z
- Files changed: 13 files
- Commits: 30 commits

Implementation approach:
## Type of Change
  <!-- Put an `x` in the boxes that apply -->

  - [ ] Bugfix
  - [x] New feature
  - [ ] Enhancement
  - [ ] Refactoring
  - [ ] Dependency updates
  - [ ] Documentation
  - [ ] CI/CD

  ### Fixes #8440

  ## Description
  <!-- Describe your changes in detail -->

  This PR implements **V2 UCS (Unified Connector Service) Integration** for authorization and synchronization flows, building upon the existing V1 UCS integration (PR#[8280](https://github.com/juspay/
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** feat(core): Hyperswitch <|> UCS integration v2 (#8439)
Co-authored-by: debarshi.gupta <debarshi.gupt
Files to modify:


**crates/external_services/src/grpc_client/unified_connector_service.rs**
  Add:
    - function: pub::payment_get

**crates/hyperswitch_domain_models/src/merchant_connector_account.rs**
  Modify existing code

**crates/router/src/core/payments.rs**
  Modify existing code

**crates/router/src/core/payments/flows.rs**
  Modify existing code

**crates/router/src/core/payments/flows/authorize_flow.rs**
  Modify existing code

**crates/router/src/core/payments/flows/psync_flow.rs**
  Add:
    - function: private::call_unified_connector_service

**crates/router/src/core/payments/transformers.rs**
  Modify existing code

**crates/router/src/core/unified_connector_service.rs**
  Modify existing code

**crates/router/src/core/unified_connector_service/transformers.rs**
  Add:
    - function: private::foreign_try_from
    - impl: impl ForeignTryFrom<&RouterData<PSync, PaymentsSyncData, PaymentsResponseData>> for payments_grpc::PaymentServiceGetRequest

**crates/router/src/types.rs**
  Modify existing code