[Synthetics] Enable synthetics e2e in flaky test runner !! (#210778)

## Summary

Enable synthetics e2e in flaky test runner !!


![image](https://github.com/user-attachments/assets/d21dd0bf-e9b7-4a18-9568-22bfe09e67fd)

.buildkite/pipelines/flaky_tests/groups.json

@@ -103,6 +103,10 @@
     {
       "key": "cypress/cloud_security_posture_serverless",
       "name": "[Serverless] Cloud Security Posture - Cypress"
+    },
+    {
+      "key": "elastic_synthetics/synthetics_plugin",
+      "name": "Synthetics plugin e2e tests - Elastic/Synthetics"
     }
   ]
-}
+}

.buildkite/pipelines/flaky_tests/pipeline.ts

@@ -193,6 +193,31 @@ for (const testSuite of testSuites) {
         },
       });
       break;
+    case 'elastic_synthetics':
+      const synthGroup = groups.find((g) => g.key === testSuite.key);
+      if (!synthGroup) {
+        throw new Error(
+          `Group configuration was not found in groups.json for the following synthetics suite: {${suiteName}}.`
+        );
+      }
+      steps.push({
+        command: `.buildkite/scripts/steps/functional/${suiteName}.sh`,
+        label: synthGroup.name,
+        agents: expandAgentQueue('n2-4-spot'),
+        key: `synthetics-suite-${suiteIndex++}`,
+        depends_on: 'build',
+        timeout_in_minutes: 30,
+        parallelism: testSuite.count,
+        concurrency,
+        concurrency_group: process.env.UUID,
+        concurrency_method: 'eager',
+        cancel_on_build_failing: true,
+        retry: {
+          automatic: [{ exit_status: '-1', limit: 3 }],
+        },
+      });
+      break;
+
     default:
       throw new Error(`unknown test suite: ${testSuite.key}`);
   }