feat(storage): support delete_source_objects in compose API

Implements the delete_source_objects parameter in Blob.compose() to allow
automatic deletion of source objects upon successful composition.

Also adds unit and system tests to verify the new parameter and handles PR feedback.

Reference PR: googleapis/google-cloud-java#12873

Co-authored-by: nidhiii-27 <224584462+nidhiii-27@users.noreply.github.com>

Author: google-labs-jules[bot]

packages/google-cloud-storage/tests/system/test_blob.py

@@ -868,13 +868,14 @@ def test_blob_compose_delete_source_objects(shared_bucket, blobs_to_delete):
 
     assert destination.download_as_bytes() == payload_1 + payload_2
 
-    if source_1.exists():
-        blobs_to_delete.append(source_1)
-    assert not source_1.exists()
-
-    if source_2.exists():
-        blobs_to_delete.append(source_2)
-    assert not source_2.exists()
+    try:
+        assert not source_1.exists()
+        assert not source_2.exists()
+    finally:
+        if source_1.exists():
+            blobs_to_delete.append(source_1)
+        if source_2.exists():
+            blobs_to_delete.append(source_2)
 
 
 def test_blob_compose_new_blob_wo_content_type(shared_bucket, blobs_to_delete):