Support attachments

[jl-wynen]

Need to find a good interface to expose attachments. One idea is:

dset = client.get_dataset(...)
assert dset.attachments is Unset  # not initialised
attachments = client.fetch_attachments(dset) # or pass pid
# or
dset = client.with_fetched_attachments(dset)
assert dset.attchments == [ ... ] # all attachments stored as a list

dset = Dataset(...)
assert dset.attachments == []
dset.attachments.append(Attachment(...))
finalized = client.upload_new_dataset_now(dset)  # uploads attachments if the list is non-empty

We can also add an argument to get_dataset to immediately download the attachments.

when downloading w/o attachments, they are Unset in order to stop users from adding attachments and getting into an inconsistent state between the client and database (e.g., client as attachment A and dbase has [B, C]).

[nitrosx]

@jl-wynen this looks good and reasonable.
I would make the attachment functionality as similar as we can to the file download one.

[jl-wynen]

Done in #135