* feature: add DiscordSocketRestClient
this resolves#803.
Users can access a DiscordSocketRestClient from the new
`DiscordSocketClient.Rest` property.
DiscordSocketRestClient is a wrapper over DiscordRestClient with certain
state-modifying methods, such as Login/Logout disabled, to prevent users
from breaking the client state.
DiscordSocketRestClient uses the same API client as the
DiscordSocketClient, allowing for shared ratelimiting - meaning users
can now force HTTP requests without needing to wory about running into
429s.
* fix: disallow users from bypassing shadowed login
