feature: Add INVITE_CREATE and INVITE_DELETE events (#1491)

* Add invite events (create and delete)

* Removed unused using

* Fixing IInviteMetadata properties

* Add two new fields to the gateway event

* Better event summary and remarks

* Change how to assign to target variable

Co-Authored-By: Joe4evr <jii.geugten@gmail.com>

* Applying suggested changes to TargetUserType

* Renaming NotDefined to Undefined

* Fixing xml docs

* Changed the summary style format

Co-authored-by: Joe4evr <jii.geugten@gmail.com>

src/Discord.Net.WebSocket/DiscordShardedClient.cs

@@ -338,6 +338,9 @@ namespace Discord.WebSocket
             client.UserIsTyping += (oldUser, newUser) => _userIsTypingEvent.InvokeAsync(oldUser, newUser);
             client.RecipientAdded += (user) => _recipientAddedEvent.InvokeAsync(user);
             client.RecipientRemoved += (user) => _recipientRemovedEvent.InvokeAsync(user);
+
+            client.InviteCreated += (invite) => _inviteCreatedEvent.InvokeAsync(invite);
+            client.InviteDeleted += (channel, invite) => _inviteDeletedEvent.InvokeAsync(channel, invite);
         }
 
         //IDiscordClient