chore: incoming webhooks and tokens migration (#5670)

https://linear.app/unleash/issue/2-1699/db-create-migration-for-a-new-incoming-webhooks-table

Adds a migration that creates 2 tables:
 - `incoming_webhooks`
 - `incoming_webhook_tokens`

src/migrations/20231215105713-incoming-webhooks.js

@@ -0,0 +1,39 @@
+exports.up = function (db, cb) {
+    db.runSql(
+        `
+        CREATE TABLE IF NOT EXISTS incoming_webhooks
+            (
+                id SERIAL PRIMARY KEY NOT NULL,
+                enabled BOOLEAN DEFAULT true NOT NULL,
+                name TEXT NOT NULL,
+                created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT now(),
+                created_by_user_id INTEGER NOT NULL
+            );
+        CREATE INDEX incoming_webhooks_enabled_idx ON incoming_webhooks(enabled);
+
+        CREATE TABLE IF NOT EXISTS incoming_webhook_tokens
+            (
+                id SERIAL PRIMARY KEY NOT NULL,
+                secret TEXT NOT NULL,
+                name TEXT NOT NULL,
+                incoming_webhook_id INTEGER NOT NULL REFERENCES incoming_webhooks(id) ON DELETE CASCADE,
+                created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT now(),
+                created_by_user_id INTEGER NOT NULL
+            );
+        CREATE INDEX incoming_webhook_tokens_webhook_id_idx ON incoming_webhook_tokens(incoming_webhook_id);
+        `,
+        cb,
+    );
+};
+
+exports.down = function (db, cb) {
+    db.runSql(
+        `
+        DROP INDEX IF EXISTS incoming_webhooks_enabled_idx;
+        DROP INDEX IF EXISTS incoming_webhook_tokens_webhook_id_idx;
+        DROP TABLE IF EXISTS incoming_webhook_tokens;
+        DROP TABLE IF EXISTS incoming_webhooks;
+        `,
+        cb,
+    );
+};