Make AuthInternal implement the interface

[Feiyang1]

• Implement the interface from @firebase/auth-interop-types
• rename the internal component to auth-internal, so consumers don't need to change their existing code for exp. (It's also easier for the typings).

FYI, @schmidt-sebastian

[changeset-bot]

⚠️ No Changeset found

Latest commit: e514ed7

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[google-oss-bot]

Binary Size Report

Affected SDKs

No changes between base commit (a9ba7ba) and head commit (699d8b4).

Test Logs

• Base (a9ba7ba7): https://github.com/firebase/firebase-js-sdk/actions/runs/329961119
• Head (699d8b4c): https://github.com/firebase/firebase-js-sdk/actions/runs/330221640