Add support for batch writes

[maitham]

Add support for firestore batch writes.

[daveah]

Any plans to work on this? I'm trying to use the framework in a project right now and it is working well, but I do need batch writes to mock my entire use case.

[kekel87]

Hello, progress on this subject ?

[flrngel]

For those who are looking for batch transaction in 2022, check https://github.com/ainbr/python-mock-firestore/pull/1/files