feat(gotrue): Implement linkIdentityWithIdToken method

[grdsdev]

What kind of change does this PR introduce?

Feature - This PR adds a new linkIdentityWithIdToken method to the GoTrue client for linking identities using ID tokens, and refactors the existing signInWithIdToken method to share common code.

What is the current behavior?

Currently, users can only sign in with ID tokens using the signInWithIdToken method, but there's no way to link additional identities to an existing user account using ID tokens. The signInWithIdToken and any potential identity linking methods would have duplicate code.

What is the new behavior?

• New Method: Added linkIdentityWithIdToken method that allows linking identities to existing users using ID tokens
• Code Refactoring: Extracted common logic into _signInWithIdToken helper method to eliminate code duplication
• Backward Compatibility: signInWithIdToken maintains the exact same API and behavior
• Shared Logic: Both methods now share validation, HTTP request handling, and session management logic

Key Features:

• Supports the same OAuth providers as signInWithIdToken (google, apple, kakao, keycloak)
• Sends link_identity: true flag when linking identities
• Proper error handling and session management
• Comprehensive documentation

Additional context

This implementation follows the same pattern as the existing signInWithIdToken method but adds the link_identity flag for identity linking functionality. The refactoring improves code maintainability by eliminating duplication while preserving all existing functionality.

The changes are minimal and focused, affecting only the GoTrue client authentication methods without any breaking changes to the public API.

Reference PR: supabase/auth-js#1096

Close #1111
Close #1200

[dshukertjr]

Huge!

[Vinzent03]

LGTM.

[grdsdev]

Thanks for the approval. However, this isn’t deployed to production yet. I’ll merge it once it’s deployed.

[erichin]

@grdsdev @dshukertjr
Thank you for adding this feature. Could you let me know when it will be released for Flutter?
Without this feature, our application cannot pass the Apple review.

[coveralls]

Pull Request Test Coverage Report for Build 17735587008

Details

• 0 of 15 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 80.42%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/gotrue/lib/src/gotrue_client.dart	0	15	0.0%

Totals
Change from base Build 17477396007:	-0.3%
Covered Lines:	3105
Relevant Lines:	3861

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💛 - Coveralls

[jerdmeyer]

Is this feature available for use in the @supabase/supabase-js client? I would love to use it in my expo react native app on iOS to support users experiencing the app first without enforcing sign in :)