[agrawalradhika-cell]

Summary

As part of the incremental migration from golang.org/x/oauth2 to cloud.google.com/go/auth (following #196), this PR introduces parallel cloud.google.com/go/auth types, helpers, and handlers in util/settings.go, util/pkce.go, and util/auth-handlers.go while keeping all existing golang.org/x/oauth2/authhandler APIs intact and marked deprecated.

Changes

• util/settings.go & util/fetch.go:
  • Added GoogleAuthHandler auth.AuthorizationHandler to Settings (with precedence documented) and marked AuthHandler authhandler.AuthorizationHandler as Deprecated: Use GoogleAuthHandler instead..
  • Updated Settings.State doc comment to reference GoogleAuthHandler (or the deprecated AuthHandler).
  • Added resolveAuthHandler() on Settings which prefers GoogleAuthHandler and falls back to converting AuthHandler (auth.AuthorizationHandler(s.AuthHandler)), and wired it into FindJSONCredentials in util/fetch.go so callers setting GoogleAuthHandler are honored immediately.
• util/pkce.go:
  • Added GeneratePKCEOptions() *auth.PKCEOptions using RFC 7636 S256 challenge generation.
  • Retained GeneratePKCEParams() *authhandler.PKCEParams as a deprecated wrapper delegating to GeneratePKCEOptions().
• util/auth-handlers.go:
  • Added Get3LOAuthHandler(...) auth.AuthorizationHandler.
  • Retained Get3LOAuthorizationHandler(...) authhandler.AuthorizationHandler as a deprecated wrapper delegating to Get3LOAuthHandler(...).
• Unit Tests:
  • Added util/settings_test.go, util/pkce_test.go, and util/auth-handlers_test.go covering resolveAuthHandler precedence, FindJSONCredentials honoring GoogleAuthHandler, GetAuthType, PKCE challenge derivation (GeneratePKCEOptions & GeneratePKCEParams), and 3LO loopback/OOB handlers (Get3LOAuthHandler & Get3LOAuthorizationHandler).

Testing

• go test -v ./util/...
• make build
• make test