feat(servers): allow setting user_data for rebuild

[phm07]

See https://docs.hetzner.cloud/changelog#2026-01-16-server-rebuild-with-user-data

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.62%. Comparing base (80f4310) to head (b85d00c).

Files with missing lines	Patch %	Lines
hcloud/servers/client.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #627      +/-   ##
==========================================
- Coverage   96.64%   96.62%   -0.03%     
==========================================
  Files          76       76              
  Lines        4029     4031       +2     
==========================================
+ Hits         3894     3895       +1     
- Misses        135      136       +1     

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