feature: Implement Dispose for types which have disposable data (#1171)

* Initial set of dispose implementations

Not handled yet:
- Discord.Net.Websocket/Entities/SocketGuild
- Discord.Net.Tests

* Refactor DiscordSocketClient init into ctor

This way we remove an IDisposableAnalyzer warning for not disposing
the client when we set the client variable.

* Dispose of clients when disposing sharded client

* Finish implementing IDisposable where appropriate

I opted to use NoWarn in the Tests project as it wasn't really necessary
considering that our tests only run once

* Tweak samples after feedback

samples/01_basic_ping_bot/Program.cs

@@ -16,21 +16,28 @@ namespace _01_basic_ping_bot
     // - https://github.com/foxbot/patek - a more feature-filled bot, utilizing more aspects of the library
     class Program
     {
-        private DiscordSocketClient _client;
+        private readonly DiscordSocketClient _client;
 
         // Discord.Net heavily utilizes TAP for async, so we create
         // an asynchronous context from the beginning.
         static void Main(string[] args)
-            => new Program().MainAsync().GetAwaiter().GetResult();
-
-        public async Task MainAsync()
         {
+            new Program().MainAsync().GetAwaiter().GetResult();
+        }
+
+        public Program()
+        {
+            // It is recommended to Dispose of a client when you are finished
+            // using it, at the end of your app's lifetime.
             _client = new DiscordSocketClient();
 
             _client.Log += LogAsync;
             _client.Ready += ReadyAsync;
             _client.MessageReceived += MessageReceivedAsync;
+        }
 
+        public async Task MainAsync()
+        {
             // Tokens should be considered secret data, and never hard-coded.
             await _client.LoginAsync(TokenType.Bot, Environment.GetEnvironmentVariable("token"));
             await _client.StartAsync();