Anonymous credentials draft endpoints

[LDiazN]

This PR adds support for the anonymous credentials protocol to the Ooni API:

1. Adds ooniauth-py as a dependency
2. Adds new endpoints: /register, /manifest, /submit

Note that the new measurement /submit endpoint won't replace the old one, and for now it's meant to be used mostly during development

New migrations

This PR will require the following database migrations:

TODO

closes #1014 #1015

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.27%. Comparing base (d943f03) to head (355e059).
⚠️ Report is 1 commits behind head on master.

❌ Your project check has failed because the head coverage (89.27%) is below the target coverage (95.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1016      +/-   ##
==========================================
- Coverage   92.81%   89.27%   -3.55%     
==========================================
  Files          17       27      +10     
  Lines        1281     2639    +1358     
  Branches       65      233     +168     
==========================================
+ Hits         1189     2356    +1167     
- Misses         78      223     +145     
- Partials       14       60      +46     

Flag	Coverage Δ
ooniauth	100.00% <ø> (?)
oonimeasurements	86.11% <ø> (?)
ooniprobe	?

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.