Remove rest package and add Rest property to the DiscordClient

Created by: IDoEverything

Here's to 5.x and another 3 year wait!

Summary

Deletes the DSharpPlus.Rest package, moves the rest client to the main package and adds a Rest property to the DiscordClient that's a DiscordRestClient initialized on startup.

First of (hopefully) many internal changes to 5.x.

Details

Fairly easy change, nothing much to it. Drafting for now, also plan to somehow make DoRequestAsync and DoMultipartAsync public to use unimplemented endpoints or endpoints with some things not implemented. Feel free to review though!