feat: allow maintainers to provide additional meta information

[robingenz]

There should be a way to provide the Capacitor Plugin Registry with additional information about a plugin that is not available via npm. This can be the case either if the information is not available on npm or if the plugin is distributed via another npm registry. Examples of such information are:

• What platforms are supported? (For example, the package @capawesome/capacitor-photo-editor only supports Android, but Android and iOS are shown).
• Is the package currently being maintained?
• Who is the author?
• Link to the GitHub repository
• Link to the installation instructions
• ...

Plugin authors could easily provide this information via JSON files (similar to the React Native Directory).

One possible scheme would be:

{
  "githubUrl": "https://github.com/capawesome-team/capacitor-plugins",
  "npmPkg": "@capawesome-team/capacitor-nfc",
  "author": "Robin Genz",
  "license": "MIT License",
  "platforms": ["android", "ios", "web"],
  "private": true,
  "unmaintained": false,
  ...
}