Support JSON output

Please, consider adding support for json output which can be enabled with a CLI option or an environment variable. It would be especially beneficial for 'gsutil ls -L gs://bucket/object/path' command which currently produces output in a custom format which is hard to parse.
BTW `aws` cli tool which provides similar functionality can produce JSON output in `aws s3api list-objects` mode (and probably does this by default)

Start by tracing the gsutil ls -L command and the existing CLI option or environment-variable handling. Define the JSON output scope for that command, then verify that the selected option or variable produces parseable JSON; the issue names no files or tests, so those entry points must be located first.

Written by the indexing model from the issue text.