[WIP] Add MediatR Extensions

[Foxtrek64]

For those who want to use MediatR. Adds a service which translates gateway events into MediatR requests. All requests return Result.

TODO:

• [x] Add GatewayEventRequest
• [x] Add implementation information (i.e., add request handlers which implement IResultRequestHandler, call AddDiscordMessaging() service provider extension)
• [ ] Determine how to differentiate between ResponderGroups.
• [x] Add Tests

[AraHaan]

Is this still being worked on?

[Nihlus]

I'm going to close this as stale unless anyone has contrary opinions. Comment here and I'll reopen.