[K8s Plugin] Implement generateVariantWorkloadManifests

[Warashi]

What this PR does:

as title

Why we need it:

It is to be used in the canary/baseline rollout like the following codes:

pipecd/pkg/app/piped/executor/kubernetes/canary.go

Lines 181 to 185 in e5ce1cc

	generatedWorkloads, err := generateVariantWorkloadManifests(workloads, configMaps, secrets, variantLabel, variant, suffix, replicasCalculator)
	if err != nil {
	return nil, err
	}
	canaryManifests = append(canaryManifests, generatedWorkloads...)

pipecd/pkg/app/piped/executor/kubernetes/baseline.go

Lines 151 to 155 in e5ce1cc

	generatedWorkloads, err := generateVariantWorkloadManifests(workloads, nil, nil, variantLabel, variant, suffix, replicasCalculator)
	if err != nil {
	return nil, err
	}
	baselineManifests = append(baselineManifests, generatedWorkloads...)

Which issue(s) this PR fixes:

Part of #5764

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 85.33333% with 11 lines in your changes missing coverage. Please review.

Project coverage is 28.05%. Comparing base (fa5b3b3) to head (7991db8).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...g/app/pipedv1/plugin/kubernetes/deployment/misc.go	85.33%	8 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5864      +/-   ##
==========================================
+ Coverage   27.97%   28.05%   +0.07%     
==========================================
  Files         510      510              
  Lines       54354    54429      +75     
==========================================
+ Hits        15204    15268      +64     
- Misses      37996    38004       +8     
- Partials     1154     1157       +3     

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[Warashi]

I'll resolve the conflicts after merging #5863

[Warashi]

I resolved the conflicts.

[ffjlabo]

Can we add t.Parallel()?

[Warashi]

Thank you, added 7991db8.

[ffjlabo]

LGTM

[Warashi]

@t-kikuc sorry, please re-review.