test(e2e): add end to end testing

[phm07]

This PR adds end to end tests. Tests are located in test/e2e/. The HCLOUD_TOKEN environment variable should contain an API token to an empty project for the tests to work.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.45%. Comparing base (74e3ae5) to head (9396f76).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
test/e2e/variables.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #785      +/-   ##
==========================================
+ Coverage   66.41%   71.45%   +5.04%     
==========================================
  Files         242      243       +1     
  Lines        8727     8731       +4     
==========================================
+ Hits         5796     6239     +443     
+ Misses       2218     1824     -394     
+ Partials      713      668      -45     

Flag	Coverage Δ
e2e	43.91% <50.00%> (?)
unit	66.41% <ø> (?)

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[apricote]

Could you move the general refactoring into a separate commit/PR?