feat: dataset tagging

[Panaetius]

adds 3 new commands:

• renku dataset tag <dataset> <tag> [-d <description>] [--force] creates a tag
• renku dataset ls-tags <dataset> List all tags in a dataset
• renku dataset rm-tags <dataset> <tags...> Remove one or more tags from a dataset

Additionally, importing a dataset tags it with the remote version.
Exporting a dataset with tags allows selecting what tag to export and temporarily checks out the repo at that tag for exporting.

The export command does not currently work for dirty directories (as that'd require stashing local changes and popping them later, which I wasn't sure was necessary), but I can add that if wanted.

Closes #566

[jsam]

Thank you so much for doing this. It's an amazing contribution and I'm super happy about having this. 👍

I made some minor comments which would help us merge this into the new structure, other than that it looks and works great.

[rokroskar]

Looks good! I tested the functionality and it seems to work as promised.