objectstore: add new skip_preserve_owner to Object.export()

This commit allows to exclude preserving ownership from an object
export. This is required to fix the issue that on macOS the an
podman based workflow cannot export objects with preserving
ownerships.

Originally this was a `no_preserve: Optional[List[str]] = None)`
to be super flexible in what we pass to `cp` but then I felt like
YAGNI - if we need more we can trivially change this (internal)
API again :)

test/mod/test_objectstore.py

@@ -320,3 +320,32 @@ def test_store_server(tmp_path):
 
         with pytest.raises(RuntimeError):
             _ = client.read_tree_at("42", tmp_path, "/nonexistent")
+
+
+@pytest.mark.skipif(os.getuid() != 0, reason="needs root")
+def test_object_export_preserves_ownership_by_default(tmp_path, object_store):
+    tree = object_store.new("a")
+    p = Path(tree, "A")
+    p.touch()
+    os.chown(os.fspath(p), 1000, 1000)
+    tree.export(tmp_path)
+
+    expected_exported_path = Path(tmp_path, "A")
+    assert expected_exported_path.exists()
+    assert expected_exported_path.stat().st_uid == 1000
+    assert expected_exported_path.stat().st_gid == 1000
+
+
+@pytest.mark.skipif(os.getuid() != 0, reason="needs root")
+def test_object_export_preserves_override(tmp_path, object_store):
+    tree = object_store.new("a")
+    p = Path(tree, "A")
+    p.touch()
+    os.chown(os.fspath(p), 1000, 1000)
+    tree.export(tmp_path, skip_preserve_owner=True)
+
+    expected_exported_path = Path(tmp_path, "A")
+    assert expected_exported_path.exists()
+    assert expected_exported_path.stat().st_uid == 0
+    assert expected_exported_path.stat().st_gid == 0
+