Add per-backend HTTP version support

[omarfawzi]

Summary

• allow selecting HTTP/1 or HTTP/2 per backend through backend/http/client config
• document HTTP version option and example usage
• update sample config to force HTTP/1 on /users
• test HTTP client creation for different versions
• move HTTP version helpers into separate file

Testing

• go test ./...

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https://chatgpt.com/codex/tasks/task_e_684bca4acdc08323a1ab171725a09f13

[codecov]

Codecov Report

Attention: Patch coverage is 28.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 18.07%. Comparing base (590a835) to head (8b3c4d5).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/lura/clients/httpclient.go	35.00%	13 Missing ⚠️
internal/lura/clients/provider.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   17.55%   18.07%   +0.52%     
==========================================
  Files          18       19       +1     
  Lines         581      603      +22     
==========================================
+ Hits          102      109       +7     
- Misses        476      491      +15     
  Partials        3        3              

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