Add label names as filter in issue search api

[adelowo]

I am working on some sort of Gitea integration that needs to comment on a list of issues
with a certain label. I don't want to manually keep track of the label ID for each repository
I am running the service against.

Sample request:

http://localhost:3000/api/v1/repos/damn/KKKKK/issues?labels=bug,duplicate

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@f21ae12). Click here to learn what that means.
The diff coverage is 80.95%.

@@           Coverage Diff            @@
##             master   #5946   +/-   ##
========================================
  Coverage          ?   38.7%           
========================================
  Files             ?     330           
  Lines             ?   48593           
  Branches          ?       0           
========================================
  Hits              ?   18809           
  Misses            ?   27062           
  Partials          ?    2722

Impacted Files	Coverage Δ
models/issue_label.go	60.74% <100%> (ø)
routers/api/v1/repo/issue.go	31.26% <69.23%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f21ae12...44870b7. Read the comment docs.