Mila extend header custom options

[milaGGL]

This PR introduces support for configuring custom API client headers via RequestOptions. It enables wrapper libraries and internal tools (like ADK) to specify their own client identifiers when making requests to the generative backend.

Changes include:

• Added a customApiClientHeader internal property to RequestOptions.
• Introduced RequestOptionsFactory.createWithCustomHeader() to allow internal tools to create a copy of RequestOptions that includes the custom header.
• Updated GenerativeModel, LiveGenerativeModel, and TemplateGenerativeModel to read customApiClientHeader and append it to their base apiClient string.

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📝 PRs merging into main branch

Our main branch should always be in a releasable state. If you are working on a larger change, or if you don't want this change to see the light of the day just yet, consider using a feature branch first, and only merge into the main branch when the code complete and ready to be released.

[google-oss-bot]

Did you forget to add a changelog entry? (Add the 'no-changelog' label to the PR or 'NO_RELEASE_CHANGE' to the description to silence this warning.)

[google-oss-bot]

The public api surface has changed for the subproject ai-logic_firebase-ai:
error: Removed constructor com.google.firebase.ai.type.RequestOptions(long,int,String) [RemovedMethod]
error: Added class com.google.firebase.ai.type.RequestOptionsFactory [AddedClass]

Please update the api.txt files for the subprojects being affected by this change by running ./gradlew ${subproject}:generateApiTxtFile. Also perform a major/minor bump accordingly.