Add plugin registry

[ffjlabo]

What this PR does:

I added the package pkg/plugin/registry.
This package is responsible for finding which plugin is used to deploy the app.

I implemented the two use-cases as methods for now.

1. GetPluginClientsByAppConfig: Get the plugin client based on the app config
2. GetPluginClientByStageName: Get the plugin client based on the stage name

These will be used in the controller, scheduler, planner like this↓ (draft implementation)
add-plugin-registry...use-plugin-registry

Why we need it:

We need to find the plugin for the app to deploy, detect the diff, and get the live state from the plugin.

At first, we added the plugin name to the model.Application on the PR #5457.
However, users should update the app to change its available plugins on the Web UI when they want to use another plugin. It takes more time for users.

So, we decided to determine the plugin used to deploy an app from the app config.

Which issue(s) this PR fixes:

Part of #5259

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):

[codecov]

Codecov Report

Attention: Patch coverage is 65.51724% with 20 lines in your changes missing coverage. Please review.

Project coverage is 26.19%. Comparing base (602bf8c) to head (58f6e2a).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/plugin/registry/registry.go	65.51%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5467      +/-   ##
==========================================
+ Coverage   26.15%   26.19%   +0.04%     
==========================================
  Files         457      458       +1     
  Lines       49111    49169      +58     
==========================================
+ Hits        12843    12881      +38     
- Misses      35246    35266      +20     
  Partials     1022     1022              

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[Warashi]

LGTM

[IMO]
We have 2 kinds of packages under pkgs/plugin, but it's confusing. So we may have to separate them.

1. packages mainly used by the plugins
2. packages mainly used by the piped, and it operates about the plugins

Let's discuss and do it on another PR.

[t-kikuc]

[ASK]
What if not ok? I think it should return an error.

Suggested change

	}
	}else{
	return nil, fmt.Errorf("no plugin found for the stage %s", stage.Name.String())
	}

instead of the following:

	if len(plugins) == 0 {
		return nil, fmt.Errorf("no plugin found for each stages")
	}

The same in getPluginClientsByNames().

[ffjlabo]

I see, thanks.
I fixed to return when the plugin couldn't find by the given identity (such as stage name or plugin name)
58f6e2a

[t-kikuc]

[ASK] Is it possible to add t.Parallel()?

Also in t.Run(tt.name, func(t *testing.T) {.

[ffjlabo]

Thanks, added on 1e1bd50

[ffjlabo]

@Warashi Sorry, plz re-approve 🙏

[t-kikuc]

Thank you!