refactor: use inheritance for endpoints

[limwa]

Refactors the endpoints to use inheritance. Useful to enforce a specific structure for all endpoints, as well as having functions that accept a generic Endpoint<T>.

Review checklist

• [x] Terms and conditions reflect the current change
• [ ] Contains enough appropriate tests
• [x] If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• [x] Properly adds an entry in changelog.md with the change
• [x] If PR includes UI updates/additions, its description has screenshots
• [x] Behavior is as expected
• [x] Clean, well-structured code

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 3.33333% with 29 lines in your changes missing coverage. Please review.

Project coverage is 11%. Comparing base (bfe1fda) to head (bf27d91). Report is 13 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1362 +/- ## ======================================= - Coverage 11% 11% -0% ======================================= Files 267 265 -2 Lines 7321 7332 +11 ======================================= - Hits 802 799 -3 - Misses 6519 6533 +14 ```