feat(auth): Adding user and idTokenResult Observables

[jamesdaniels]

Checklist

• Issue number for this PR: AngularFireAuth idToken emits a string instead of User #1464
• Docs included?: yes
• Test units included?: yes
• In a clean directory, yarn install, yarn test run successfully? yes

Description

Adding user and idTokenResult to AngularFireAuth.

[markstuart]

Looks good to me, I like that the user subscription will fire for all auth events and that you are recommending its use instead of authState.

👍

[davideast]

LGTM!