Added new header for routing to desired database

[KIRTIKUMARKK21]

Description

Added a new DATABASE_ID_HEADER_KEY constant (x-database-id) to RequestContextConstants to support database routing decisions in multi-database deployments. This header allows services to specify which database instance should handle a request, enabling database sharding or multi-tenancy at the database level.

The header is:

• Added to HEADER_PREFIXES_TO_BE_PROPAGATED to ensure it propagates across gRPC service calls
• Added to CACHE_MEANINGFUL_HEADERS since database selection affects query results and must be considered for cache invalidation

Testing

• Verified the constant compiles without errors
• Confirmed the header is included in propagation and cache-meaningful header sets
• Manual testing required: Deploy and verify the header propagates correctly in a multi-service environment
• Manual testing required: Verify cache behavior respects the database ID header

Checklist:

• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

Documentation

No documentation changes required - this is an internal constant addition. Services consuming this library will need to document their own usage of the x-database-id header based on their specific database routing implementation.

[puneet-traceable]

I caught up with QE as well and for them to be able to test, they would need a way to use both(mongo/pg) for the same tenant. I am thinking should we use two flags on the lines( usePostgres/forceUsePostgres).
@anujgoyal1 Your thoughts?

[aaron-steinfeld]

I'd love to hear more about what you're trying to accomplish with this and see if we can find a way to do so that doesn't break abstraction like this. Could we set up some time to discuss (or just ping me)