Add stat function to datasets

[Matt-Haugh]

This PR adds a stat function to datasets which mimics some of the functionality of https://docs.python.org/3/library/os.html#os.stat.

[acroz]

Hey @Matt-Haugh, thanks for the contribution!

My first thought is that os.stat gives information about a single file, whereas what you're trying to do here is more like ls -l - getting extra info about a list of files.

Would it make sense to instead integrate this into ls above? Looking at the ls (i.e. the command line tool) documentation, -l means 'long format', which we could borrow here with a signature like:

def ls(prefix="/", project_id=None, long=False, show_hidden=False, object_client=None):

What do you think?

Aside from resolving this question, this PR will also need to implement tests, but we can maybe discuss this once the above question is resolved.

[Matt-Haugh]

Cheers @acroz, that sounds sensible. In fact part of the motivation for adding this was to add ls -l to the datasets branch that @liamcoatman has for faculty-cli. I'll get on reworking this over the next few days