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/sql/20230617-webhooks.sql

@@ -0,0 +1,9 @@
+-- +migrate Up
+CREATE TABLE IF NOT EXISTS discord_webhooks (
+    id TEXT NOT NULL PRIMARY KEY,
+    webhook_id TEXT NOT NULL,
+    webhook_token TEXT NOT NULL
+);
+
+-- +migrate Down
+DROP TABLE discord_webhooks;