feature: add DiscordSocketRestClient (#1198)

* 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

src/Discord.Net.WebSocket/DiscordSocketRestClient.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Threading.Tasks;
+using Discord.Rest;
+
+namespace Discord.WebSocket
+{
+    public class DiscordSocketRestClient : DiscordRestClient
+    {
+        internal DiscordSocketRestClient(DiscordRestConfig config, API.DiscordRestApiClient api) : base(config, api) { }
+
+        public new Task LoginAsync(TokenType tokenType, string token, bool validateToken = true)
+            => throw new NotSupportedException("The Socket REST wrapper cannot be used to log in or out.");
+        internal override Task LoginInternalAsync(TokenType tokenType, string token, bool validateToken)
+            => throw new NotSupportedException("The Socket REST wrapper cannot be used to log in or out.");
+        public new Task LogoutAsync()
+            => throw new NotSupportedException("The Socket REST wrapper cannot be used to log in or out.");
+        internal override Task LogoutInternalAsync()
+            => throw new NotSupportedException("The Socket REST wrapper cannot be used to log in or out.");
+    }
+}