refactor: add generic API request helper functions

[jooola]

Those helpers function will be used to replace a lot of boilerplate using in clients.

See the tests to have a better idea of how it can be used.

For now, no API clients are refactored to use those helpers, this will happen incrementally in future PRs.

[codecov]

Codecov Report

Attention: Patch coverage is 43.10345% with 33 lines in your changes missing coverage. Please review.

Project coverage is 67.58%. Comparing base (51baf8d) to head (1427945).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
hcloud/client_generic.go	43.10%	22 Missing and 11 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #572      +/-   ##
==========================================
- Coverage   67.87%   67.58%   -0.29%     
==========================================
  Files          47       48       +1     
  Lines        4912     4970      +58     
==========================================
+ Hits         3334     3359      +25     
- Misses       1170     1192      +22     
- Partials      408      419      +11     

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