Add example of datastore with v2 functions. (GoogleCloudPlatform#11096)

## Description

Add example for Cloud Functions and Firestore in Datastore Mode

## Checklist
- [X] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md)
- [X] README is updated to include [all relevant information](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#readme-file)
- [ ] **Tests** pass:   `nox -s py-3.9` (see [Test Environment Setup](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/.github/CODEOWNERS) with the codeowners for this sample
- [ ] This sample adds a new **Product API**, and I updated the [Blunderbuss issue/PR auto-assigner](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/.github/blunderbuss.yml) with the codeowners for this sample
- [x] Please **merge** this PR for me once it is approved

Author: JU-2094

functions/v2/datastore/hello-datastore/main.py

@@ -0,0 +1,40 @@
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the 'License');
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an 'AS IS' BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# [START functions_cloudevent_datastore]
+from cloudevents.http import CloudEvent
+import functions_framework
+from google.events.cloud import datastore
+
+
+@functions_framework.cloud_event
+def hello_datastore(cloud_event: CloudEvent) -> None:
+    """Triggers by a change to a Firestore entity.
+
+    Args:
+        cloud_event: cloud event with information on the firestore event trigger
+    """
+    datastore_payload = datastore.EntityEventData()
+    datastore_payload._pb.ParseFromString(cloud_event.data)
+
+    print(f"Function triggered by change to: {cloud_event['source']}")
+
+    print("\nOld value:")
+    print(datastore_payload.old_value)
+
+    print("\nNew value:")
+    print(datastore_payload.value)
+
+
+# [END functions_cloudevent_datastore]

functions/v2/datastore/hello-datastore/main_test.py

@@ -0,0 +1,54 @@
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the 'License');
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an 'AS IS' BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from cloudevents.http import CloudEvent
+from google.events.cloud import datastore
+
+import main
+
+
+def test_hello_datastore(capsys):
+    old_entity = datastore.EntityResult(
+        entity=datastore.Entity(
+           properties={"name": datastore.Value(string_value="Old Test Name")}),
+    )
+    new_entity = datastore.EntityResult(
+        entity=datastore.Entity(
+           properties={"name": datastore.Value(string_value="New Test Name")}),
+    )
+
+    datastore_payload = datastore.EntityEventData(
+        old_value=old_entity,
+        value=new_entity,
+    )
+
+    attributes = {
+        "id": "5e9f24a",
+        "type": "google.cloud.datastore.entity.v1.written",
+        "source": "/projects/test-project/databases/(default)/documents/test-kind/test-entity",
+    }
+
+    data = datastore_payload._pb.SerializeToString()
+
+    cloud_event = CloudEvent(attributes, data)
+
+    # Calling the function with the mocked cloud event
+    main.hello_datastore(cloud_event)
+
+    out, _ = capsys.readouterr()
+
+    assert "\nOld value:" in out
+    assert datastore_payload.old_value.__str__() in out
+    assert "\nNew value:" in out
+    assert datastore_payload.value.__str__() in out

functions/v2/datastore/hello-datastore/requirements-test.txt

@@ -0,0 +1 @@
+pytest==7.4.4

functions/v2/datastore/hello-datastore/requirements.txt

@@ -0,0 +1,6 @@
+functions-framework==3.5.0
+google-events==0.11.0
+google-cloud-datastore==2.19.0
+google-api-core==2.15.0
+protobuf==4.25.2
+cloudevents==1.10.1