[k8s plugin] Add custom unmarshal logic for k8s plugin configs

[ffjlabo]

What this PR does:

SSIA

Why we need it:

We want to set the default values for configs.

Which issue(s) this PR fixes:

Part of #5764

Does this PR introduce a user-facing change?:

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

[khanhtc1202]

👌

[codecov]

Codecov Report

❌ Patch coverage is 0% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.55%. Comparing base (1a0e3c2) to head (5490b7d).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...pp/pipedv1/plugin/kubernetes/config/application.go	0.00%	12 Missing ⚠️
.../pipedv1/plugin/kubernetes/config/deploy_target.go	0.00%	12 Missing ⚠️
pkg/app/pipedv1/plugin/kubernetes/config/plugin.go	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6167      +/-   ##
==========================================
- Coverage   28.56%   28.55%   -0.02%     
==========================================
  Files         535      536       +1     
  Lines       57288    57321      +33     
==========================================
+ Hits        16364    16367       +3     
- Misses      39652    39683      +31     
+ Partials     1272     1271       -1     

Flag	Coverage Δ
.	23.13% <ø> (+<0.01%)	⬆️
.-pkg-app-pipedv1-plugin-analysis	62.85% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	60.32% <0.00%> (-0.59%)	⬇️
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.48% <ø> (ø)
.-pkg-app-pipedv1-plugin-scriptrun	54.83% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	38.31% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-waitapproval	68.62% <ø> (ø)
.-pkg-plugin-sdk	50.32% <ø> (+0.07%)	⬆️
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[Warashi]

👍🏻

[khanhtc1202]

👌