test(session): load tests for session api (#9212)

# Which Problems Are Solved

We currently are not able to benchmark the performance of the session
api

# How the Problems Are Solved

Load tests were added to
- use sessions in oidc tokens analog
https://zitadel.com/docs/guides/integrate/login-ui/oidc-standard

# Additional Context

- Closes https://github.com/zitadel/zitadel/issues/7847

load-test/src/membership.ts

@@ -0,0 +1,25 @@
+import http from 'k6/http';
+import { Trend } from 'k6/metrics';
+import url from './url';
+import { check, fail } from 'k6';
+
+const addIAMMemberTrend = new Trend('membership_iam_member', true);
+export async function addIAMMember(userId: string, roles: string[], accessToken: string): Promise<void> {
+  const res = await http.post(
+    url('/admin/v1/members'),
+    JSON.stringify({
+      userId: userId,
+      roles: roles,
+    }),
+    {
+      headers: {
+        authorization: `Bearer ${accessToken}`,
+        'Content-Type': 'application/json',
+      },
+    },
+  );
+  check(res, {
+    'member added successful': (r) => r.status == 200 || fail(`unable add member: ${JSON.stringify(res)}`),
+  });
+  addIAMMemberTrend.add(res.timings.duration);
+}