External Chat Linking (!4)

This PR adds the functionality for plugins to send and recieve messages linked to a specific channel.

Co-authored-by: Layla <layla@layla.gg>
Reviewed-on: https://gitea.sumulayla.synology.me/layla/birdbot/pulls/4

common/external_chat.go

@@ -0,0 +1,11 @@
+package common
+
+type ExternalChatManager interface {
+	SendMessage(user string, message string)
+}
+
+type ExternalChatModule interface {
+	Initialize(ExternalChatManager)
+
+	RecieveMessage(user User, message string)
+}