Allow adding custom plugin registry

[khakers]

Is your feature request related to a problem? Please elaborate.

Some users that host multiple bots, or bots for others, may want to maintain control over allowed plugins but also add others without undergoing the lengthy or impossible process of merging into the modmail plugin registry.

Describe the solution you'd like

potential implementations

1. Allow setting a registry override env variable, should be a URL providing json in registry format e.g. "PLUGIN_REGISTRY_OVERRIDE_URL" or "PLUGIN_REGISTRY_URL"
2. An option to add a new registry source which is merged with the default
   both could be implemented, though 1 would be easiest.

Does your solution involve any of the following?

• Logviewer
• New config option

Describe alternatives you've considered

No change

Who will this benefit?

Advanced users
modmail hosts

Additional Information

No response