feat(dataconnect): add "X-Firebase-Sqlconnect-Affinity" header for GSLB soft stickiness

[dconeybe]

This PR adds the X-Firebase-Sqlconnect-Affinity request header to outgoing Data Connect HTTP requests. This header enables backend server affinity routing for SQL Connect to improve server resource usage efficiency and performance.

Highlights

• SQL Connect Server Affinity Header: Added the X-Firebase-Sqlconnect-Affinity header containing ${projectId}${serviceId} to Data Connect HTTP request headers.
• Unit Test Coverage: Updated EXPECTED_HEADERS, EXPECTED_HEADERS_WITH_GEN, and EMULATOR_EXPECTED_HEADERS test fixtures to verify the header key and formatted value.

Changelog

• data-connect-api-client-internal.ts
  • Added GetHeadersParameters interface and updated getHeaders function to accept parameters by name.
  • Added X-Firebase-Sqlconnect-Affinity header containing ${projectId}${serviceId} to request headers.
  • Updated makeGqlRequest to resolve projectId and pass named parameters to getHeaders.
• data-connect-api-client-internal.spec.ts
  • Added X-Firebase-Sqlconnect-Affinity header assertion to EXPECTED_HEADERS, EXPECTED_HEADERS_WITH_GEN, and EMULATOR_EXPECTED_HEADERS.

[gemini-code-assist]

Code Review

This pull request introduces the 'X-Firebase-Sqlconnect-Affinity' header to the Data Connect API client, constructed from the project ID and service ID. The review feedback recommends using a delimiter (such as ':') between the project ID and service ID to avoid potential key collisions and ambiguity, and suggests updating the corresponding unit tests to reflect this change.