feat(Client): Support Proxy

- https://github.com/discordjs/discord.js/issues/3039#issuecomment-1033955220
- #283
(not tested yet)

Co-Authored-By: Nguyễn Hồng Đức <hongducyb123@gmail.com>

src/client/Client.js

@@ -897,6 +897,9 @@ class Client extends BaseClient {
     if (options && typeof options.patchVoice !== 'boolean') {
       throw new TypeError('CLIENT_INVALID_OPTION', 'patchVoice', 'a boolean');
     }
+    if (options && typeof options.proxy !== 'string') {
+      throw new TypeError('CLIENT_INVALID_OPTION', 'proxy', 'a string');
+    }
     if (typeof options.shardCount !== 'number' || isNaN(options.shardCount) || options.shardCount < 1) {
       throw new TypeError('CLIENT_INVALID_OPTION', 'shardCount', 'a number greater than or equal to 1');
     }