Add support for R-GCIP tenant configuration

[srushtisv]

Description

This PR introduces support for Regional GCIP (R-GCIP) within the Firebase Auth iOS SDK. Developers can now initialize an Auth instance tied to a specific tenant ID and location.

The key changes are:

• New TenantConfig Struct: A Sendable struct TenantConfig has been added to hold the tenantId and location (defaulting to "prod-global").
• New Auth Factory Method: A new static method Auth.auth(app: FirebaseApp, tenantConfig: TenantConfig) allows initialization with R-GCIP parameters.
• Updated AuthRequestConfiguration: The internal AuthRequestConfiguration class now includes location and tenantId properties, populated during initialization if a TenantConfig is provided.

These changes enable applications to work with regionalized Firebase Authentication backends.

Changelog

• [FirebaseAuth] - Added support for Regional GCIP (R-GCIP) by introducing TenantConfig and a new Auth initializer to specify tenant ID and location.

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[google-oss-bot]

	1 Warning
⚠️	Did you forget to add a changelog entry? (Add #no-changelog to the PR description to silence this warning.)

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[ncooke3]

LGTM with the one code suggestion above.

[pashanka]

LGTM, apart from one comment.