[Firebase] Add snapshot cursors sample (#1780)

* Add snapshot cursors sample

* Updating sample with a test

Author: frankyn

firestore/cloud-client/snippets.py

@@ -557,6 +557,22 @@ def cursor_simple_end_at():
     return query_end_at
 
 
+def snapshot_cursors():
+    db = firestore.Client()
+    # [START fs_start_at_snapshot_query_cursor]
+    doc_ref = db.collection(u'cities').document(u'SF')
+
+    snapshot = doc_ref.get()
+    start_at_snapshot = db.collection(
+        u'cities').order_by(u'population').start_at(snapshot)
+    # [END fs_start_at_snapshot_query_cursor]
+    results = start_at_snapshot.limit(10).get()
+    for doc in results:
+        print('{}'.format(doc.id))
+
+    return results
+
+
 def cursor_paginate():
     db = firestore.Client()
     # [START cursor_paginate]

firestore/cloud-client/snippets_test.py

@@ -195,6 +195,14 @@ def test_cursor_simple_end_at():
     snippets.cursor_simple_end_at()
 
 
+def test_snapshot_cursors(capsys):
+    snippets.snapshot_cursors()
+    out, _ = capsys.readouterr()
+    assert "SF" in out
+    assert "TOK" in out
+    assert "BJ" in out
+
+
 def test_cursor_paginate():
     snippets.cursor_paginate()