No custom claim support for Firebase Auth

**Is your feature request related to a problem? Please describe.** I am trying to extract custom claims from the JWT I get back in ``` await app.auth().verifyIdToken( token, checkRevoked: true, ); ``` There appears to be no support for custom claims **Describe the solution you'd like** Custom claims support for a map of Any values **Describe alternatives you've considered** Using JWT libraries to decode the token **Additional context** None.