WIP Firestore: replace gax and regenerate gapic

[chemelnucfin]

Toward #4692

@jonparrott I haven't ran the tests yet, but something like this?

It looks like it's removed some enums that wasn't replaced. Is that correct behavior?

[chemelnucfin]

@jonparrott I have fixed the unit tests, but I haven't figured out what's causing the 504 deadline_exceeded errors in the system tests. There're also some gax stuff in gapic/firestore_admin_client.py that I had to edit manually, which didn't seem right.

I'll probably head back to pubsub again and then come back to this. If you see something obvious that I'm doing wrong, please let me know. thanks.

[chemelnucfin]

I also looked at the config files, but think that the commits should not be idempotent. I did also try to retry the nonidempotent ones with deadline_exceeded though.

[theacodes]

@chemelnucfin no worries. Thanks for giving this a shot but pubsub is absolutely a higher priority.

[chemelnucfin]

Closing for now