Firestore : add emulator support

[HemangChothani]

Fixes: #7500

[crwilcox]

Have we validated our tests pass against the emulator?

[HemangChothani]

The Doc testing requires modifications to be made to environment_vars.py file found in the Core library; namely extending the emulator variable declarations by another variable. During the test, however, the core library gets overwritten through pip install, losing the changes made to environment_vars.py and hence the newly added variable, which results in the doc test failure.

[tseaver]

The failed system test is due to #7130.

@HemangChothani To fix the docs build, you could update firestore/noxfile.py to install the "local dependencies", inside the docs session, e.g.:

    for local_dep in LOCAL_DEPS:
        session.install("-e", local_dep

[tseaver]

@HemangChothani

I don't see a response to the question @crwilcox asked:

Have we validated our tests pass against the emulator?

[crwilcox]

@HemangChothani I am closing this because I ended up implementing this PR #8721 and forgot you had started on this. There are 4 open bugs internally against the emulator for small differences I found when running the tests against the emulator