[SDK] add unit types to use in config types

[Warashi]

What this PR does:

as title

Why we need it:

They are used generally in the platform provider's specific configs in pipedv0, and are useful to define config types.
And I want to use them in the Kubernetes plugin.

In the current implementation of the Kubernetes plugin, we use pkg/configv1, but it's not a good way.

pipecd/pkg/app/pipedv1/plugin/kubernetes/config/baseline.go

Lines 17 to 25 in f39124f

	import config "github.com/pipe-cd/pipecd/pkg/configv1"
	// K8sBaselineRolloutStageOptions contains all configurable values for a K8S_BASELINE_ROLLOUT stage.
	type K8sBaselineRolloutStageOptions struct {
	// How many pods for BASELINE workloads.
	// An integer value can be specified to indicate an absolute value of pod number.
	// Or a string suffixed by "%" to indicate an percentage value compared to the pod number of PRIMARY.
	// Default is 1 pod.
	Replicas config.Replicas `json:"replicas"`

Which issue(s) this PR fixes:

Part of #5530

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 91.48936% with 8 lines in your changes missing coverage. Please review.

Project coverage is 28.60%. Comparing base (f39124f) to head (d7dfc5f).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/plugin/sdk/unit/replicas.go	89.13%	4 Missing and 1 partial ⚠️
pkg/plugin/sdk/unit/duration.go	86.36%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5959      +/-   ##
==========================================
+ Coverage   28.50%   28.60%   +0.10%     
==========================================
  Files         520      523       +3     
  Lines       56434    56528      +94     
==========================================
+ Hits        16084    16170      +86     
- Misses      39083    39089       +6     
- Partials     1267     1269       +2     

Flag	Coverage Δ
.	23.80% <ø> (ø)
.-pkg-app-pipedv1-plugin-example	0.00% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	66.42% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	50.97% <91.48%> (+2.11%)	⬆️
.-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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[khanhtc1202]

It's a bit hard to decide whether we should include this in the SDK or keep these plugins separate and accept the fact that they could be redundant and copied around.

I think what is placed in the SDK should be something more like piped care about, not something plugins need. Since we can say it's common, but it may block/confuse other plugins that don't want to use the SDK-defined config structure. WDYT?

[Warashi]

Yes, I also think it's hard to decide whether we should put these in the SDK. I noticed we already have a " diff " package; it's not something piped cares about, but a package we thought was commonly used across plugins.
IMO, we can propose some best practices for plugin developers about the config structure and include the use of this unit package.

[khanhtc1202]

@Warashi I got your point, then for now I only concern for the name of package, but it's not too strong. Let's keep it as is for now 👍

[khanhtc1202]

LGTM 👍

[t-kikuc]

thanks, wait stage needs it too.