Add Unknown event

[bakirov-eldar]

I propose adding a new event handler for Unknown Event for unimplemented and/or undocumented events.

For example: there has long been an undocumented GatewayEvent such as GUILD_JOIN_REQUEST_UPDATE. There are no guarantees that it will exist permanently, however, I can use it at my own risk with the DiscordNet library.

This will allow not to change the DiscordNet code when developing a bot.