feat: support using client credentials with emulator

[paulharter]

This changes the grpc.Channel used when connecting to a local emulator so that the client credentials are used. This allows for testing database rules against the emulator in python.

Fixes ##268

thanks

[BenWhitehead]

This seems like a convenient thing to have, what do you think @crwilcox ?

[crwilcox]

Thanks for the contribution @paulharter. In order for us to accept this we would need it to pass linting and coverage. The linting can be fixed by running nox -s blacken and verifying with nox -s lint. If you look at the kokoro build you will see what lines weren't covered, google/cloud/firestore_v1/base_client.py 151 7 38 0 96% 178-199

[paulharter]

@crwilcox I've added a test and linted, I hope it's ok now.

Thank you for accepting this, it will make testing much easier.

[crwilcox]

Thanks for the contribution @paulharter. This will go out in the next minor release of the library.