Refactor the way UserCredentials work in anticipation of mudularization

[sam-gc]

Basically:
Instead of interpreting the result of the IdTokenResponse as soon as it comes in, we delay that until the user directly requests it.

Before:

const userCred = await signInWithPopup(...);
const accessToken = userCred.credential.accessToken;

After:

const userCred = await signInWithPopup(...);
const credential = GoogleAuthProvider.credentialFromResult(userCred);
const accessToken = credential.accessToken;

This saves pulling in at least four different unnecessary classes

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Latest commit: e1ce16f

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[google-oss-bot]

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Affected SDKs

No changes between base commit (e2efd27) and head commit (af23e80).

Test Logs

• Base (e2efd271): https://github.com/firebase/firebase-js-sdk/actions/runs/181693762
• Head (af23e804): https://github.com/firebase/firebase-js-sdk/actions/runs/186159005