Add license file to official plugins

[xellos00]

Checklist

• [ ] New Feature for the SVC/Plugin
• [ ] New Feature for CI/CD
• [ ] Enhancement on Vatz

• [x] Others(etc. e.g, documentation,...)

Please describe, what it's about or related Problem with

A clear description of what feature need to be developed or enhanced in Vatz

There's no license file in official plugins. I think it's good to have in plugins as well as VATZ

Describe the output that you are expecting for above

A clear and concise description of what you expect to happen.

Describe alternatives you've considered

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Additional context

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[xellos00]

• https://pkg.go.dev/github.com/dsrvlabs/vatz-plugin-sysutil
• https://pkg.go.dev/github.com/dsrvlabs/vatz-plugin-cosmoshub

[heejin-github]

Add License file

• https://github.com/dsrvlabs/vatz-plugin-cosmoshub/pull/33
• https://github.com/dsrvlabs/vatz-plugin-sysutil/pull/12