chore: action_set_events db table migration (#6298)

https://linear.app/unleash/issue/2-1962/implement-new-action-events-logic

Adds a new `action_set_events` table for the new action events logic.

Even though observable events are technically immutable, we're storing
the observable event along with the action set event. This allows us to
avoid 1 join while allowing us to persist action set event information
after deleting observable events, if we wish to do so at a later stage.

src/migrations/20240221082758-action-events.js

@@ -0,0 +1,31 @@
+exports.up = function (db, cb) {
+    db.runSql(
+        `
+        CREATE TABLE IF NOT EXISTS action_set_events
+        (
+            id SERIAL PRIMARY KEY NOT NULL,
+            action_set_id INTEGER NOT NULL,
+            observable_event_id INTEGER NOT NULL,
+            created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT now(),
+            state TEXT NOT NULL,
+            observable_event JSONB NOT NULL,
+            action_set JSONB NOT NULL
+        );
+
+        CREATE INDEX IF NOT EXISTS idx_action_set_events_observable_event_id ON action_set_events(observable_event_id);
+        CREATE INDEX IF NOT EXISTS idx_action_set_events_action_set_id_state ON action_set_events(action_set_id, state);
+        `,
+        cb,
+    );
+};
+
+exports.down = function (db, cb) {
+    db.runSql(
+        `
+        DROP INDEX IF EXISTS idx_action_set_events_observable_event_id;
+        DROP INDEX IF EXISTS idx_action_set_events_action_set_id_state;
+        DROP TABLE IF EXISTS action_set_events;
+        `,
+        cb,
+    );
+};