Add RetroAchievements plugin

[LuanRoger]

I've created the RetroAchievements plugin for metrics, using the RetroAchievements's official API.
It follows the style and layout of the Steam plugin, and here it's what I have done so far. I'm opening the PR just to track the checks or feedback.

I've seen that metrics will launch the 4.0 soon, but since there's no release date to it, the changes could be merged and I will do the migrations by my self when the new version's out, fixing any breaking changes and migrating other aspects to fit the new specs.

Render output

This output was generated by my metrics action, using this brench as reference.

Ready to merge?

• I have correctly filled metadata.yml
  • name is set with an unused emoji and plugin name
  • category is set to community
  • examples contains links towards a rendered output hosted by you
  • authors contains my GitHub username
  • supports list which account type are supported
  • scopes are correctly listed with their associated names on GitHub (leave an empty array if not applicable)
  • inputs are correctly filled
• I have implemented my plugin
  • index.mjs respects the plugins guidelines
  • Hide achievements when showachievements is no
  • Implement the template (only classic is available)
• I have tested my plugin locally
  • tests/mocks ... have been created
  • app.placeholder.js has been updated for preview from web instances
  • examples.yml contains workflows examples (at least one is required)
    • skip: true has been set for prod attribute in each test
  • npm run linter yields no errors
• I have documented my plugin
  • README.md eventually describes complex setup or options (if applicable)
  • Show how to get the Web API Key