Adding Missing Guild Widget Methods (#650)

Created by: spydacarnage

Summary

Resolves the missing methods listed in #650

Details

• Added new methods to:
  • DiscordGuild.cs
  • DiscordApiClient.cs
  • DiscordRestClient.cs
• Added new consts to:
  • Endpoints.cs
• Added new payload class to:
  • RestGuildPayloads.cs
• Added 3 new Entities:
  • DiscordWidgetSettings
  • DiscordWidget
  • DiscordWidgetMember