feat: add limit-conn plugin form

[LiteSun]

Please answer these questions before submitting a pull request, or your PR will get closed.

Why submit this pull request?

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What changes will this PR take into?
Add limit-conn plugin form

Related issues

Checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[codecov-io]

Codecov Report

Merging #1728 (ce925d0) into master (990662a) will decrease coverage by 10.26%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #1728       +/-   ##
===========================================
- Coverage   62.55%   52.29%   -10.27%     
===========================================
  Files         123       38       -85     
  Lines        5253     2660     -2593     
  Branches      649        0      -649     
===========================================
- Hits         3286     1391     -1895     
+ Misses       1779     1081      -698     
  Partials      188      188               

Flag	Coverage Δ
backend-unit-test	52.29% <ø> (ø)
frontend-e2e-test	?

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

Impacted Files	Coverage Δ
web/src/pages/Service/components/Step1.tsx
web/src/pages/Upstream/transform.ts
...pages/Route/components/Step1/MatchingRulesView.tsx
web/src/helpers.tsx
web/src/pages/SSL/components/Step1/index.tsx
...rc/components/PluginOrchestration/customConfig.tsx
web/src/pages/User/Login.tsx
web/src/components/Upstream/constant.ts
...pages/SSL/components/CertificateUploader/index.tsx
web/src/pages/Upstream/components/Step1.tsx
... and 74 more

---

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 990662a...ce925d0. Read the comment docs.

[netlify]

Deploy preview for apisix-dashboard ready!

Built with commit 9afc7f5

https://deploy-preview-1728--apisix-dashboard.netlify.app