[K8s Plugin] Refactor to use LoadConfigSpec to load application config

[Warashi]

What this PR does:

This PR does a refactoring to use sdk.LoadConfigSpec instead of configv1.DecodeYAML

Why we need it:

We don't want to import the configv1 package under plugin implementation because it is not a package for plugin implementation.

Which issue(s) this PR fixes:

Part of #5530
Follows #5731

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 46.87500% with 17 lines in your changes missing coverage. Please review.

Project coverage is 26.96%. Comparing base (b6c89b1) to head (09b1c1a).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
...app/pipedv1/plugin/kubernetes/deployment/plugin.go	20.00%	7 Missing and 1 partial ⚠️
...p/pipedv1/plugin/kubernetes/deployment/rollback.go	66.66%	1 Missing and 2 partials ⚠️
...g/app/pipedv1/plugin/kubernetes/deployment/sync.go	70.00%	1 Missing and 2 partials ⚠️
.../app/pipedv1/plugin/kubernetes/livestate/plugin.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5733   +/-   ##
=======================================
  Coverage   26.96%   26.96%           
=======================================
  Files         503      503           
  Lines       53024    53023    -1     
=======================================
+ Hits        14297    14298    +1     
+ Misses      37663    37660    -3     
- Partials     1064     1065    +1     

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[Warashi]

I implemented config unmarshalling before invoking the plugin implementation in #5740, so this is no longer needed.