add referrers api

[wy65701436]

Thank you for contributing to Harbor!

Comprehensive Summary of your change

Issue being fixed

Fixes #(issue)

Please indicate you've done the following:

• 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

Merging #18324 (a7a58e7) into main (ab636fe) will increase coverage by 0.00%.
The diff coverage is 58.49%.

@@           Coverage Diff            @@
##             main   #18324    +/-   ##
========================================
  Coverage   67.47%   67.47%            
========================================
  Files         970      971     +1     
  Lines      106117   106223   +106     
  Branches     2649     2649            
========================================
+ Hits        71600    71674    +74     
- Misses      30690    30712    +22     
- Partials     3827     3837    +10     

Flag	Coverage Δ
unittests	67.47% <58.49%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/lib/patterns.go	100.00% <ø> (ø)
...rc/server/middleware/artifactinfo/artifact_info.go	85.71% <ø> (ø)
src/server/middleware/metric/metric.go	0.00% <ø> (ø)
src/server/registry/route.go	0.00% <0.00%> (ø)
src/server/registry/referrers.go	61.38% <61.38%> (ø)

... and 7 files with indirect coverage changes

[stonezdj]

LGTM

[chlins]

lgtm