fix(auth): handle non-Transport DefaultTransport

[adapap]

If a client does not specify opts.BaseRoundTripper in httptransport.NewClient, avoid a panic if the global http.DefaultTransport has been overwritten to something that is not an instance of *http.Transport.

Our use case is to create a new Google API client, but the constructor for NewClient does not allow passing a BaseRoundTripper option as seen here.

Fixes #10742.

[codyoss]

@adapap Can you please create an issue for discussing the change, thanks.

[adapap]

@codyoss Based on our discussion in #10742, I've updated this PR to export BaseTransport from the auth/internal/transport package, and use this as the fallback instead of accepting the http.DefaultTransport if it was modified.

[codyoss]

@adapap Thank you for your contribution!