Prepare Plugin SDK

[Warashi]

What this PR does:

This PR adds an SDK for plugin implementation.
The implementation is partial, and I'll send other PRs to implement the rest.

Why we need it:

There are many boilerplates for implementing plugins.

What I want to be reviewed:

The API designs.

The users of this SDK will write codes like the one below.

package main

import (
  ...
)

func main() {
  pipedsdk.RegisterDeploymentPlugin(&pluginImpl{})
  if err := pipedsdk.Run(); err != nil {
     log.Fatal(err) // or some error handling
  }
}

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 0% with 159 lines in your changes missing coverage. Please review.

Project coverage is 26.33%. Comparing base (cfe6136) to head (e6ebd92).
Report is 34 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/plugin/pipedsdk/sdk.go	0.00%	115 Missing ⚠️
pkg/plugin/pipedsdk/deployment.go	0.00%	44 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5529      +/-   ##
==========================================
- Coverage   26.53%   26.33%   -0.20%     
==========================================
  Files         464      467       +3     
  Lines       49917    50066     +149     
==========================================
- Hits        13243    13183      -60     
- Misses      35615    35828     +213     
+ Partials     1059     1055       -4     

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[Warashi]

The Config parameter is the plugin's config defined in piped's config.

[t-kikuc]

Let's add a comment to explain that 👍

[Warashi]

I added config on this commit.
337e8c8

[t-kikuc]

Let's add a comment to explain that 👍

[t-kikuc]

LGTM

Let's sophisticate the SDK through implementing plugins.

[Warashi]

I updated the copyright notice year to 2025 since these files were added this year.

[ffjlabo]

Thank you. LGTM