SAML Provider

## What feature would you like to see?
Firebase Auth SAML Provider support
https://cloud.google.com/identity-platform/docs/how-to-enable-application-for-saml . Firebase auth is set up for SAML providers, but the Android SDK doesn't include one. And trying to use the OAuthProvider returns an exception: `An internal error has occurred. [ INVALID_CREDENTIAL_OR_PROVIDER_ID:Invalid IdP response/credential:`

## How would you use it?

Without it, the SDK is incomplete and SAML provider implementation isn't possible within a Firebase project, so I believe it's required (unless you only authenticate via the web)