[FEATURE] allow updating connector_customer in batch PM update API

[kashif-m]

Feature Description

Support updating connector_customer field via batch payment method update API. When connector_customer_id is provided in CSV with merchant_connector_ids, store it in customer's connector_customer field and return in response.

Possible Implementation

Request:

• Add connector_customer_id: Option<String> to UpdatePaymentMethodRecord

Response:

• Add connector_customer: Option<pii::SecretSerdeValue> to response structures

Logic:

• Fetch and cache MCA data once

• Update customer's connector_customer with key format:

  • Payout processors: {profile_id}_{connector_name}
  • Others: merchant_connector_id

• Return updated connector_customer in response

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

• I checked and didn't find a similar issue

Have you read the Contributing Guidelines?

• I have read the Contributing Guidelines

Are you willing to submit a PR?

Yes, I am willing to submit a PR!

[kashif-m]

Re-opening for a hotfix.