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

persistence/database.go

@@ -4,4 +4,12 @@ package persistence
 type Database interface {
 	GetDiscordMessage(id string) (string, error)
 	SetDiscordMessage(id string, messageID string) error
+
+	GetDiscordWebhook(id string) (*DBDiscordWebhook, error)
+	SetDiscordWebhook(id string, data *DBDiscordWebhook) error
+}
+
+type DBDiscordWebhook struct {
+	ID    string
+	Token string
 }