feat(metadata): add user agent header

[jooola]

• Move version to dedicated package, to break a cyclic import.
• Add a user agent header to the metadata client.
• Extract user agent utility to an internal package.

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.28%. Comparing base (692e551) to head (0d68a79).

Files with missing lines	Patch %	Lines
hcloud/metadata/client.go	40.00%	3 Missing ⚠️
hcloud/client.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #866      +/-   ##
==========================================
- Coverage   78.40%   78.28%   -0.12%     
==========================================
  Files          59       60       +1     
  Lines        4366     4370       +4     
==========================================
- Hits         3423     3421       -2     
- Misses        640      646       +6     
  Partials      303      303              

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