Preserve the order of column schema properties in `schema.yaml` files. (#3923)

When using `bqetl query schema update` to create a new `schema.yaml` file, BigQuery returns the column schema properties in a sensible order (`name`, `type`, `mode`, `fields`), but our `schema.yaml` output has been sorting those properties alphabetically which makes it much less readable.

Also, when using `bqetl query schema update` to update an existing `schema.yaml` file, this will now preserve whatever order the column schema properties were in.

bigquery_etl/schema/__init__.py

@@ -270,7 +270,7 @@ class Schema:
     def to_yaml_file(self, yaml_path: Path):
         """Write schema to the YAML file path."""
         with open(yaml_path, "w") as out:
-            yaml.dump(self.schema, out, default_flow_style=False)
+            yaml.dump(self.schema, out, default_flow_style=False, sort_keys=False)
 
     def to_json_file(self, json_path: Path):
         """Write schema to the JSON file path."""