Allow installing plugins declaratively

[Infinidoge]

Description

Implements the ability for PeerTube to self-install plugins that are declared upfront in a file, checking the file on startup to install/uninstall/update accordingly. The format for the declarative_plugins.json file is as such:

{
  "npm-name-here": {
    "pluginPath": "path/to/plugin/directory",
    // OR
    "version": "yarn.add.compatible-version",
    
    "extraArgs": "--extra --arguments --passed --to --yarn",
    "preInstall": "command run before calling yarn",
    "postInstall": "command run after calling yarn"
  }
}

This design makes it very flexible for different ways of handling installing/uninstaling, with the only thing left out being a way to specify a different command to install a plugin with (such as using npm instead of yarn). Since Yarn is the foundation for plugin management internally, I decided to leave that out in this implementation.

I am not a seasoned TypeScript developer by any stretch of the imagination, so please let me know wherever the code can be improved.

TODO:

• Add usage to documentation
• Add unit test

Related issues

Resolves #6428

Has this been tested?

• 👍 yes, I added tests to the test suite
• 💭 no, because this PR is a draft and still needs work
• 🙅 no, because this PR does not update server code
• 🙋 no, because I need help