Issue:
feat: add env variable for enable key manager service

## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [x] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
This PR adds env variable for enabling encryption service. Application changes for encryption service is not backward compatible, so once the build is deployed and certified stable this feature will be enabled by setting up "true" to key_manager.enabled flag. Later only application starts using encryption service for encryption and decryption. Once the config is enabled it shouldn't be disabled.


### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [x] This PR modifies application configuration/environment variables


## Motivation and Context
This PR adds env variable for enabling encryption service. Application changes for encryption service is not backward compatible, so once the build is deployed and certified stable this feature will be enabled by setting up "true" to key_manager.enabled flag. Later only application starts using encryption service for encryption and decryption.


## How did you test it?
 **This can't be tested in Test environment, since it is already enabled in both**
1. key_manager.enabled: false and feature "encryption_service" disabled. Encryption and decryption happens within the application
2. key_manager.enabled: true and feature "encryption_service" disabled. Encryption and decryption happens within the application
3. key_manager.enabled: false and feature "encryption_service" enabled. Encryption and decryption happens within the application
4. key_manager.enabled: true and feature "encryption_service" enabled. Encryption and decryption happens with encryption service. Once this is enabled can't be disabled.

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code


Resolution:
<think>
PR Context:
- Title: feat: add env variable for enable key manager service
- Merged: 2024-07-29T11:23:54Z
- Files changed: 7 files
- Commits: 1 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

## Description
This PR adds env variable for enabling encryption service. Application changes for encryption service is not backward compatible, so once the build is deployed and certified stable this feature will be enabled by setting up "true" to key_manager.enabled flag. Later only application st
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** feat: add env variable for enable key manager service (#5465)
Co-authored-by: dracarys18 <karthikey.
Files to modify:


**crates/common_utils/src/types/keymanager.rs**
  Modify existing code

**crates/hyperswitch_domain_models/src/type_encryption.rs**
  Add:
    - function: private::is_encryption_service_enabled

**crates/router/src/configs/defaults.rs**
  Modify existing code

**crates/router/src/configs/settings.rs**
  Modify existing code

**crates/router/src/routes/admin.rs**
  Modify existing code

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