test(integration): add the vertex service-account helper the safeguards wire test imports

Backport prerequisite for #43955 on stable/1.103.x.
Copied from tests/integration/_support/vertex.py on main (added by #43900).

tests/integration/_support/vertex.py

@@ -0,0 +1,31 @@
+from __future__ import annotations
+
+import json
+from typing import Final
+
+from cryptography.hazmat.primitives import serialization
+from cryptography.hazmat.primitives.asymmetric import rsa
+
+
+def service_account_json(project: str, token_url: str) -> str:
+    private_key: Final = (
+        rsa.generate_private_key(public_exponent=65537, key_size=2048)
+        .private_bytes(
+            serialization.Encoding.PEM,
+            serialization.PrivateFormat.PKCS8,
+            serialization.NoEncryption(),
+        )
+        .decode()
+    )
+    return json.dumps(
+        {
+            "type": "service_account",
+            "project_id": project,
+            "private_key_id": "scripted",
+            "private_key": private_key,
+            "client_email": f"scripted@{project}.iam.gserviceaccount.com",
+            "client_id": "0",
+            "auth_uri": f"{token_url}/_oauth/authorize",
+            "token_uri": f"{token_url}/_oauth/token",
+        }
+    )