Change appcheck activate() to use provider pattern

[hsubox76]

Create provider classes that users can use in activate(). To avoid breaking changes, activate still takes a string site key or our previous custom provider pattern, which is any object with a getToken property.

I also changed addTokenListener to check for existing tokens in both memory and storage because otherwise it will only check storage if getToken() is called, which only happens if the refresher starts, which only happens if automatic token refresh is enabled. I don't think fetching a token from local storage should be gated on that.

Note: API changes should also be propagated to v9 compat after this.

Edit: For docs review - only need to review packages/firebase/index.d.ts

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🦋 Changeset detected

Latest commit: c9c364d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@firebase/app-check	Minor
@firebase/app-check-types	Minor
firebase	Minor
@firebase/rules-unit-testing	Patch

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Test Logs

• Base (c8ac613): https://github.com/firebase/firebase-js-sdk/actions/runs/1012428333
• Head (f199147): https://github.com/firebase/firebase-js-sdk/actions/runs/1083036025