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.

src/Discord.Net.Rest/DiscordRestClient.cs

@@ -24,6 +24,8 @@ namespace Discord.Rest
         /// </summary>
         /// <param name="config">The configuration to be used with the client.</param>
         public DiscordRestClient(DiscordRestConfig config) : base(config, CreateApiClient(config)) { }
+        // used for socket client rest access
+        internal DiscordRestClient(DiscordRestConfig config, API.DiscordRestApiClient api) : base(config, api) { }
 
         private static API.DiscordRestApiClient CreateApiClient(DiscordRestConfig config)
             => new API.DiscordRestApiClient(config.RestClientProvider, DiscordRestConfig.UserAgent);

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -43,6 +43,8 @@ namespace Discord.WebSocket
         private DateTimeOffset? _statusSince;
         private RestApplication _applicationInfo;
 
+        /// <summary> Provides access to a REST-only client with a shared state from this client. </summary>
+        public DiscordSocketRestClient Rest { get; }
         /// <summary> Gets the shard of of this client. </summary>
         public int ShardId { get; }
         /// <summary> Gets the current connection state of this client. </summary>
@@ -124,6 +126,7 @@ namespace Discord.WebSocket
             AlwaysDownloadUsers = config.AlwaysDownloadUsers;
             HandlerTimeout = config.HandlerTimeout;
             State = new ClientState(0, 0);
+            Rest = new DiscordSocketRestClient(config, ApiClient);
             _heartbeatTimes = new ConcurrentQueue<long>();
 
             _stateLock = new SemaphoreSlim(1, 1);

src/Discord.Net.WebSocket/DiscordSocketRestClient.cs

@@ -0,0 +1,16 @@
+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.");
+        public new Task LogoutAsync()
+            => throw new NotSupportedException("The Socket REST wrapper cannot be used to log in or out.");
+    }
+}