Add tags to DVC experiments

[legendof-selda]

with dvc experiments, allow the option to add tags to it so that we can easily group/filter them easily!

1. Creating an experiment dvc exp run --name "name" --tag "tag1","tag2".
2. For existing experiments dvc exp tag --name "name" --tag "tag3","tag4"
3. To remove tags dvc exp tag --name "name --tag "tag4" --untag
4. For filtering dvc exp list --tag "tag1","tag2", dvc exp list --not-tag "tag1","tag2"

This makes it very easy to use. Would be very useful especially when we need to clean up the experiments.

[lefos99]

I really like the idea of

2.For existing experiments dvc exp tag --name "name" --tag "tag3","tag4"

Because in my team, we use tags/releases to persist our models.