Add User-Agent to all request

[stonezdj]

set user agent to avoid some registry (e.g. docker hub) return 403 when user agent is not set to harbor-registry-client
fixes #23025

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Comprehensive Summary of your change

Issue being fixed

Fixes #(issue)

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• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.02%. Comparing base (1c7d831) to head (71692d7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pkg/registry/auth/authorizer.go	42.85%	3 Missing and 1 partial ⚠️
src/pkg/reg/adapter/aliacr/openapi.go	0.00%	2 Missing ⚠️
src/server/registry/manifest.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #23030   +/-   ##
=======================================
  Coverage   66.02%   66.02%           
=======================================
  Files        1073     1073           
  Lines      116495   116503    +8     
  Branches     2939     2939           
=======================================
+ Hits        76912    76919    +7     
  Misses      35329    35329           
- Partials     4254     4255    +1     

Flag	Coverage Δ
unittests	66.02% <50.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/common/utils/utils.go	82.23% <100.00%> (+0.18%)	⬆️
src/pkg/registry/auth/bearer/authorizer.go	64.63% <100.00%> (+0.43%)	⬆️
src/pkg/registry/client.go	45.04% <100.00%> (ø)
src/server/registry/manifest.go	68.15% <0.00%> (ø)
src/pkg/reg/adapter/aliacr/openapi.go	0.00% <0.00%> (ø)
src/pkg/registry/auth/authorizer.go	45.12% <42.85%> (-0.34%)	⬇️

... and 7 files with indirect coverage changes

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[chlins]

lgtm