Implement LoadPlainYAMLManifests and LoadManifests

[Warashi]

What this PR does:

• add below implementations of the k8s plugin
  • Implement LoadPlainYAMLManifests
  • Implement LoadManifests partially

Why we need it:

• We have to load k8s manifests to determine SYNC_STRATEGY, and do other k8s operations.

Which issue(s) this PR fixes:

Part of #4980

Does this PR introduce a user-facing change?: No

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

Attention: Patch coverage is 46.61017% with 63 lines in your changes missing coverage. Please review.

Project coverage is 24.36%. Comparing base (23ca1b3) to head (7372110).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...g/app/pipedv1/plugin/kubernetes/provider/loader.go	44.33%	52 Missing and 7 partials ⚠️
...app/pipedv1/plugin/kubernetes/provider/resource.go	66.66%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5284      +/-   ##
==========================================
+ Coverage   24.31%   24.36%   +0.04%     
==========================================
  Files         440      442       +2     
  Lines       46784    46902     +118     
==========================================
+ Hits        11374    11426      +52     
- Misses      34507    34566      +59     
- Partials      903      910       +7     

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[khanhtc1202]

LGTM 👍

[ffjlabo]

LGTM