feat: implement schema API

[fgksgf]

Please answer these questions before submitting a pull request

• Why submit this pull request?

• Bugfix

• New feature provided

• Improve performance

• Backport patches

• Related issues

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New feature or improvement

• Implement schema API for getting plugin's schema from APISIX.

[codecov-commenter]

Codecov Report

Merging #601 (9cbad13) into master (5635652) will increase coverage by 0.40%.
The diff coverage is 44.38%.

@@            Coverage Diff             @@
##           master     #601      +/-   ##
==========================================
+ Coverage   34.05%   34.46%   +0.40%     
==========================================
  Files          55       57       +2     
  Lines        5544     5722     +178     
==========================================
+ Hits         1888     1972      +84     
- Misses       3430     3504      +74     
- Partials      226      246      +20     

Impacted Files	Coverage Δ
pkg/ingress/apisix_cluster_config.go	0.00% <0.00%> (ø)
pkg/ingress/controller.go	1.07% <0.00%> (ø)
pkg/apisix/nonexistentclient.go	44.44% <28.57%> (-1.99%)	⬇️
pkg/apisix/cluster.go	27.20% <31.00%> (+1.87%)	⬆️
pkg/apisix/apisix.go	67.50% <50.00%> (ø)
pkg/apisix/schema.go	58.13% <58.13%> (ø)
pkg/apisix/cache/memdb.go	75.00% <76.47%> (+0.15%)	⬆️
pkg/apisix/plugin.go	80.00% <80.00%> (ø)
... and 1 more

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[fgksgf]

It's weird, all unit tests passed on my PC but failed in CI.

[tokers]

@fgksgf Please check out the CI failure problems, especially for the unit test cases.

[gxthrj]

It's weird, all unit tests passed on my PC but failed in CI.

UT passed.

[tao12345666333]

This PR need rebase.