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.Rest/BaseDiscordClient.cs

@@ -65,7 +65,7 @@ namespace Discord.Rest
             }
             finally { _stateLock.Release(); }
         }
-        private async Task LoginInternalAsync(TokenType tokenType, string token, bool validateToken)
+        internal virtual async Task LoginInternalAsync(TokenType tokenType, string token, bool validateToken)
         {
             if (_isFirstLogin)
             {
@@ -118,7 +118,7 @@ namespace Discord.Rest
             }
             finally { _stateLock.Release(); }
         }
-        private async Task LogoutInternalAsync()
+        internal virtual async Task LogoutInternalAsync()
         {
             if (LoginState == LoginState.LoggedOut) return;
             LoginState = LoginState.LoggingOut;

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);