Add support for mocking server timestamp

[AndrasTimar]

I would like to propose the use of the clock package, which would make it possible to mock the server timestamp. Now the code uses Timestamp.now(), which in return uses DateTime.now(), which can not be changed from the outside.

[ming-chu]

Hi @AndrasTimar , would you mind share more information about the clock package?

[AndrasTimar]

Hi @AndrasTimar , would you mind share more information about the clock package?

Sure, you can read about it here: https://pub.dev/packages/clock

[atn832]

That sounds great! Would you be able to contribute a PR for it?

[ming-chu]

I created a PR for this.
#287

Sorry, I am late........very late.

[atn832]

Thanks to @ming-chu for implementing this. I just published it to https://pub.dev/packages/fake_cloud_firestore/changelog#246.