Issue:
feat(core): implemented platform merchant account

## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [X] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
#### About the feature
- This the first PR for the Platform Account feature.
- Feature is under development and behind the feature flag. 
- Feature is supposed to be disabled for sandbox and production environments.
#### Enabling the platform account.
- New API has been created to enable platform account.
- API requires Admin API auth.
- Once enabled, there is no way of disabling platform account as of now.
#### How it works
- A new special header is to be sent by the platform called `x-connected-merchant-id`.
- Platform will use its own API key irrespective of the merchant id present in the above header.
- Value of this header is supposed to be the merchant id of the merchant account on behalf of which the platform is performing the operation.
- **If the platform has `is_platform` true and the merchant account on behalf of which operation is performed belongs to the same organisation as platform** then the operation is allowed.
#### Payment Intent
- Operation will be performed for merchant id present in the `x-connected-merchant-id` header.
- `platform_merchant_id` column for the payment intent will be populated with platforms merchant id.
#### Other operations
- Will add context of `platform_merchant_id` in other operations as well.

<!-- Describe your changes in detail -->


### Additional Changes

- [x] This PR modifies the API contract
- [x] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

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## Motivation and Context
#6883 
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## How did you test it?
1. Create a merchant account(m1).
2. Use below api to convert it into platform account.
```sh
curl --location --request POST '<BASE URL>/accounts/{merchant_id}/platform' \
--header 'api-key:<ADMIN API KEY>'
```
3. Create another merchant account in the same organisation(m2).
4. Do a payment using platform's(m1) API key for connected merchant account(m2) by passing header `x-connected-merchant-id` with value as connected merchant account's merchant id.
5. Check if payment is created for connected merchant account.
6. Check if `platform_merchant_id` is populated with platform's merchant id.
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## 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
- [ ] I added unit tests for my changes where possible


Resolution:
<think>
PR Context:
- Title: feat(core): add platform_account creation API 
- Merged: None
- Files changed: 13 files
- Commits: 20 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
#### Precursor PR 
#6882 (This PR has the context of platform merchant account and initial code changes).
#### Enabling the platform account.
- New API has been created to enable platform account.
- API requires `JWTAuth` with `OrganizationAccountWrite` permissions.
- Once enable
</think>

To implement this, you'll need to modify:


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