Plugin improvements

[Nutomic]

Performance:

• to improve performance dont call function_exists every time, but store a vec with all available plugin hooks
• load plugins immediately on startup, so they can be initialized in the background while lemmy initializes the db, runs migrations etc
• Load plugin metadata in parallel https://github.com/LemmyNet/lemmy/blob/9d98c82ec30e8f261fd9459a4e75dc0f979de0eb/crates/api/api_utils/src/plugins.rs#L113

New features (there are a lot of possibilities):

• move plugin configuration to a database table and support live reloading
• add plugin ui for admins to lemmy-ui with:
  • list of installed plugins
  • config for each plugin
  • list of available plugins to install (hosted on join-lemmy.ml or federated)
  • plugin stats (which hooks active, how often called, how long execution time)
• add more plugin hooks
• http endpoints like GET /api/v4/plugin/*name*/ are forwarded to plugin hook http_call(req: HttpRequest): HttpResponse
• plugins can make db queries via host function, eg for vote analysis (https://github.com/LemmyNet/lemmy/pull/5669)