Add Firestore support for the new standard GAE Python3

[fili]

NDB is currently not supported on the new standard GAE Python3, instead it prefers now Firestore.

https://github.com/authomatic/authomatic/tree/master/authomatic/extras/gae currently is limited to NDB and therefor does not work on new standard GAE Python3.

Here is an example on how to get webapp2 running on the new standard GAE Python3.

Also the Users API is not supported anymore on the new standard GAE Python3, which impacts: https://github.com/authomatic/authomatic/blob/master/authomatic/providers/gaeopenid.py

[mrichar1]

After several years of inactivity, authomatic is now under community management, and we have just released a new stable version (1.0.0).

We are now reviewing all issues and PRs and hoping to begin work to solve as many of these as possible.

We are keen to find out which issues still apply, and which PRs are still required/are likely to merge cleanly into the current code. We are aiming to review them all, but any help with prioritisation would be very useful!

If you are still interested in having this issue/PR resolved, or are able to help us work on it, please reply to this message. That way we know which issues are most important to the community.

[fili]

Yes, this is very important for the future of authomatic.