feat: Add optional audience for http-monitors via the oauth2 client credentials flow (#5950)

Co-authored-by: Frank Elsinga <frank@elsinga.de>

db/knex_migrations/2025-06-24-0000-add-audience-to-oauth.js

@@ -0,0 +1,12 @@
+exports.up = function (knex) {
+    return knex.schema
+        .alterTable("monitor", function (table) {
+            table.string("oauth_audience").nullable().defaultTo(null);
+        });
+};
+
+exports.down = function (knex) {
+    return knex.schema.alterTable("monitor", function (table) {
+        table.string("oauth_audience").alter();
+    });
+};