FR: Support sign_in_attributes for SAML

### [REQUIRED] Step 2: Describe your environment
* Library version: v4.15.0
* Firebase Product: auth (auth, database, storage, etc)

### [REQUIRED] Step 3: Describe the problem
We are providing user login using SAML with Firebase Auth.

Since we obtain and use the SAML provider's user attributes from the ID token, we would like to use the `firebase.sign_in_attributes` claim.
However, `sign_in_attributes` is not supported in `firebase-admin-go` .
Could you please make `sign_in_attributes` available?