feat(Rest): expose https.Agent options (#6177)

Co-authored-by: Antonio Román <kyradiscord@gmail.com>

src/util/Options.js

@@ -80,10 +80,18 @@
  * sent in the initial guild member list, must be between 50 and 250
  */
 
+/**
+ * HTTPS Agent options.
+ * @typedef {Object} AgentOptions
+ * @see {@link https://nodejs.org/api/https.html#https_class_https_agent}
+ * @see {@link https://nodejs.org/api/http.html#http_new_agent_options}
+ */
+
 /**
  * HTTP options
  * @typedef {Object} HTTPOptions
  * @property {number} [version=9] API version to use
+ * @property {AgentOptions} [agent={}] HTTPS Agent options
  * @property {string} [api='https://discord.com/api'] Base url of the API
  * @property {string} [cdn='https://cdn.discordapp.com'] Base url of the CDN
  * @property {string} [invite='https://discord.gg'] Base url of invites
@@ -127,6 +135,7 @@ class Options extends null {
         version: 9,
       },
       http: {
+        agent: {},
         version: 9,
         api: 'https://discord.com/api',
         cdn: 'https://cdn.discordapp.com',