Add galaxy v3 support, refactor, other fixes

[briantist]

Related #102
Related #99 (as of this PR we now override proxied requests to always request 100 results, but we still don't support true pagination).

Fixes #23
Fixes #103
Fixes #100

Large PR to add galaxy v3 support.

There's a bunch of other small fixes, enhancements, and changes here too.

There's a fair amount of refactoring. I wanted to do even more, and maybe come up with a more generalized view system so there won't be so much repetition between v2 and v3 but it was taking too long, and I need to get working v3 support out with the public galaxy switchover, so maybe that will happen another time.

This will add support both for serving over v3 and proxying upstream v3.

Proxying requests from v2 API client calls to upstream v3 or vice-versa is not supported. No translation will be done; the client request's API version must be supported by the upstream you choose.

This should end up ok because galaxy v3 is supported all the way back to ansible-galaxy in 2.9. I don't think I'll ever implement proxy translation, but possible ways to do that:

• use a real galaxy client in the upstream functionality instead of URL substitution
  • I considered taking a dependency on ansible-core and using its galaxy client (not supported, probably not a good idea); I'm not sure if it would actually be useful or provide the right methods either
  • I considered taking a dependency on antsibull-core and using its custom galaxy client, but it's all async and galactory is not (yet) which makes it kind of a pain to use; even still I'm not sure if provides the right methods
• Do a lot of conditional translation in both the URL substitution and response handling, making this stuff much more complicated (creating our own client, reinventing the wheel yet again)
• Support multiple upstreams (per api version? fallbacks? etc)

[codecov]

Codecov Report

Attention: 160 lines in your changes are missing coverage. Please review.

Comparison is base (d1af360) 56.77% compared to head (96541ae) 61.58%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   56.77%   61.58%   +4.81%     
==========================================
  Files          27       32       +5     
  Lines        1011     1463     +452     
==========================================
+ Hits          574      901     +327     
- Misses        437      562     +125     

Files	Coverage Δ
galactory/api/__init__.py	100.00% <100.00%> (ø)
galactory/api/v2/collection_imports.py	100.00% <100.00%> (ø)
galactory/api/v3/__init__.py	100.00% <100.00%> (ø)
galactory/api/v3/collection_imports.py	100.00% <100.00%> (ø)
galactory/constants.py	100.00% <100.00%> (ø)
galactory/models.py	100.00% <100.00%> (ø)
tests/unit/client/test_api.py	100.00% <100.00%> (ø)
tests/unit/models/conftest.py	100.00% <100.00%> (ø)
tests/unit/models/test_collectioncollection.py	100.00% <100.00%> (ø)
tests/unit/models/test_collectiondata.py	100.00% <100.00%> (ø)
... and 8 more

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