Add basic support for GET /observations v2

[JWCook]

Updates #155

[codecov]

Codecov Report

Merging #490 (c85ea4c) into main (c5c14ec) will increase coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head c85ea4c differs from pull request most recent head 2372ba3. Consider uploading reports for the commit 2372ba3 to get more accurate results

@@            Coverage Diff             @@
##             main     #490      +/-   ##
==========================================
+ Coverage   95.13%   95.20%   +0.06%     
==========================================
  Files          57       59       +2     
  Lines        3127     3171      +44     
  Branches      596      602       +6     
==========================================
+ Hits         2975     3019      +44     
  Misses        113      113              
  Partials       39       39              

Impacted Files	Coverage Δ
pyinaturalist/docs/templates.py	100.00% <ø> (ø)
pyinaturalist/v1/observations.py	96.61% <ø> (ø)
pyinaturalist/constants.py	100.00% <100.00%> (ø)
pyinaturalist/models/observation.py	96.42% <100.00%> (+0.23%)	⬆️
pyinaturalist/request_params.py	98.27% <100.00%> (ø)
pyinaturalist/v2/__init__.py	100.00% <100.00%> (ø)
pyinaturalist/v2/observations.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes