feat(gotrue): add custom OAuth providers admin API with custom_claims_allowlist

[spydon]

What

Implements the custom OAuth/OIDC providers admin API in gotrue, exposed as client.auth.admin.customProviders, mirroring the admin.customProviders API in supabase-js (/admin/custom-providers). This is distinct from the existing OAuth 2.1 server clients API (admin.oauth).

Resolves SDK-1173.

The parity issue asked to add custom_claims_allowlist to the custom providers admin types, but that API did not exist in the Flutter SDK at all, so this adds the full surface including the field.

Changes

• New GoTrueAdminCustomProvidersApi with listProviders({type}), createProvider, getProvider, updateProvider, deleteProvider.
• Wired into GoTrueAdminApi as admin.customProviders.
• New types: CustomProviderType, OIDCDiscoveryDocument, CustomOAuthProvider, CreateCustomProviderParams, UpdateCustomProviderParams.
• All three provider types carry customClaimsAllowlist (serialized as custom_claims_allowlist), an opt-in allowlist of raw identity provider claim keys copied verbatim into the user's custom_claims. Optional, absent by default.

Usage

await supabase.auth.admin.customProviders.createProvider(
  CreateCustomProviderParams(
    providerType: CustomProviderType.oidc,
    identifier: 'custom:mycompany',
    name: 'My Company',
    clientId: '...',
    clientSecret: '...',
    customClaimsAllowlist: ['groups', 'org_id', 'mail'],
  ),
);

Tests

• Unit tests for serialization/deserialization of custom_claims_allowlist (present, absent, empty) across the create/update/read types.
• Integration tests covering create/list/list-by-type/get/update/delete against a live gotrue server.

Integration tests use OAuth2 providers (OIDC creation triggers a server-side discovery-document fetch, which would be network-dependent). They do not assert customClaimsAllowlist round-trips through the server, since the auth image bundled with the pinned Supabase CLI accepts the field but does not yet echo it in responses; serialization is covered by the unit tests.

[Vinzent03]

I actually already vibe coded this api locally for a project right when the api came out 2 months ago, because I needed it for a project, but never took the time to polish it and create the pr 😅

[spydon]

I actually already vibe coded this api locally for a project right when the api came out 2 months ago, because I needed it for a project, but never took the time to polish it and create the pr 😅

Sounds like you're the perfect person to review this then 😁