GOATS-743: Extend API to support fetching observations from GPP.

[davner]

Jira Ticket: GOATS-743

Checklist

• Added or reviewed a release note in doc/changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@762526d). Learn more about missing BASE report.
⚠️ Report is 92 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #332   +/-   ##
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  Coverage        ?   74.77%           
=======================================
  Files           ?      208           
  Lines           ?     6851           
  Branches        ?      459           
=======================================
  Hits            ?     5123           
  Misses          ?     1643           
  Partials        ?       85           

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