[java-firestore] Provide a test double or emulator for App Engine for native firestore mode

AppEngine has an amazing set of local test doubles/emulators for all its external APIs located in

`com.google.appengine.tools.development.testing.*`

Including Datastore. But there isn't one for Firestore. Will there be?

I supposed I could build one but it's one of the reasons I use AppEngine that these test doubles work so well and have never failed me when going to a production environment.

Start by reviewing the App Engine testing entry points under `com.google.appengine.tools.development.testing.*` and how native Firestore mode is used. Define whether the work should provide a Firestore emulator or a test double, then establish compatibility expectations and tests that demonstrate local behavior matches production.

Written by the indexing model from the issue text.