FR: Support sign_in_attributes for SAML

[SpringMT]

trafficstars

[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?

[SpringMT]

How about this issue?