Add internal support for regionalized OIDC token exchange (BYO-CIAM)

[srushtisv]

Add request/response RPCs for regional OIDC token exchange feat(Auth)

Description

This PR introduces the internal request and response structures needed to support exchanging a third-party OIDC ID token for a Firebase ID token, specifically targeting the regionalized Google Cloud Identity Platform (R-GCIP) endpoints. This is a key part of the Bring Your Own CIAM (BYO-CIAM) feature.

Changes:

• ExchangeTokenRequest.swift: New struct conforming to AuthRPCRequest.
  • Constructs the complete URL for the exchangeOidcToken endpoint, dynamically including the region and tenant ID from AuthRequestConfiguration.
  • Supports both staging and production regionalized hosts.
  • Includes idToken and idpConfigID as parameters.
  • Throws a fatal error if location, tenantId, or projectID are missing in the configuration, as these are critical for this regionalized flow.
• ExchangeTokenResponse.swift: New struct conforming to AuthRPCResponse.
  • Parses the idToken (as firebaseToken) and expiresIn fields from the JSON response.
  • Calculates the token expirationDate.

This PR lays the internal groundwork for the new OIDC token exchange feature for regionalized tenants (R-GCIP). To enable the Firebase Auth iOS SDK to communicate with the new regionalized identityplatform.googleapis.com service for exchanging tokens from external OIDC providers. This requires constructing specific URLs based on tenant and location, which these new structures handle. These are internal additions to the networking layer and do not change the public API surface themselves. They will be used by higher-level functions (to be added/updated in subsequent PRs) that will expose the OIDC token exchange functionality to developers.

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