Create a new Plugin API

[bartfeenstra]

Proposals

• Allow some plugins to be created through project configuration. E.g. site owners will want to support custom event types, roles, etc
• Add PluginRepository. Which is generic over the plugin type. Subclasses for specific plugin types may add additional functionality.
  • get all plugins of a given subtype
• Plugin.name(Localizer) -> str
  • incorporate get_entity_type_name()
• Plugin.label(Localizer) -> str
  • incorporate UserFacingEntity.entity_type_label()
• Plugin.description(Localizer) -> str
• Implement for
  • Extension
  • Entity
  • Event type
  • Presence role
  • Command provider
• Require all plugins to be registered as entrypoints
• add a test fixture that sideloads plugins (by patching the API?)

Blocked by

• The 0.4.x branch being opened