[firebase_core] v2 embedding API

[bparrishMines]

Description

Fixes flutter/flutter#41869

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[cyanglaz]

LGTM! Left some comments. We should update version before merging in.

[collinjackson]

LGTM. Thanks for including an integration test!

If you add an example/test_driver/firebase_core_e2e_test.dart, CI will eventually run the integration test on iOS also. But this isn't hooked up yet, so it's optional.

[mklim]

Code itself LG, just some comments on AndroidX/Gradle changes.

[mklim]

LGTM modulo one Javadoc nit.