feat(auth): Add IdTokenResult and User::GetIdTokenResult support

[AustinBenoit]

Description

Provide details of the change, and generalize the change in the PR title above.

Adds IdTokenResult struct and User::GetIdTokenResult(bool force_refresh) to retrieve Firebase Auth ID token details, including standard claims, custom claims (std::map<Variant, Variant>), authentication/issued-at/expiration timestamps, and sign-in provider / second-factor metadata.

Key changes:

• Added IdTokenResult struct and User::GetIdTokenResult(), User::GetIdTokenResultLastResult(), and User::GetIdTokenResultThreadSafe() to firebase/auth/user.h.
• Desktop: Implemented User::GetIdTokenResult using EnsureFreshToken and a JWT payload decoder with URL-safe base64 decoding and JSON parsing into Variant maps.
• Android: Implemented User::GetIdTokenResult via JNI calling FirebaseUser.getIdToken(forceRefresh).
• iOS: Implemented User::GetIdTokenResult using [FIRUser getIDTokenResultWithCompletion:].
• Added unit tests for JWT parsing, custom/standard claims extraction, and edge cases (IdTokenResultTest in auth/tests/id_token_result_test.cc and auth/tests/user_test.cc).
• Added documentation sample in auth/samples/src/doc_samples.cc.
• Updated release notes in release_build_files/readme.md.

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

• Built firebase_auth desktop library locally.
• Built and ran firebase_auth_id_token_result_test (12 unit test cases passing, covering valid tokens with standard & custom claims, base64url padding variants, malformed tokens, and equality operators).
• Ran repository code formatting script (scripts/format_code.py -git_diff).

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[gemini-code-assist]

Code Review

This pull request adds the IdTokenResult struct and the User::GetIdTokenResult() method to retrieve decoded ID token claims and metadata across Desktop, Android, and iOS platforms. It also introduces JWT parsing utilities and comprehensive tests. The review feedback suggests explicitly deleting the local JNI reference j_token in the Android implementation to prevent reference table overflow, and logging warnings on Desktop if token parsing fails to assist with debugging.