feat: tags

[kerry-he]

What this PR does / why we need it

Allows tags to be specified as a list of strings, e.g.,

alphatrion.experiment.craft_experiment.CraftExperiment.start(name="experiment", tags=["foo", "bar"])

I will add a separate PR to account for this in the dashboard UI.

Which issue(s) this PR fixes

Fixes #209

Special notes for your reviewer

Does this PR introduce a user-facing change?

Experiments can now be tagged with lists of strings.

[kerthcet]

Thanks @kerry-he quoted the offline discussion here for transparency:

maybe let's have a separate tags table, because I don't know how labels will evolve in the future, 
it could be immutable, it could be separated to different kinds of labels, system labels and user labels 
to avoid abuse, since we're developing from scratch, and have a dedicated tags table is not a big deal, 
maybe let's start with a new table, it won't be a mistake decision in the future anyway. 
We'll make lables immutable and tags mutable today.WDYT?

[kerry-he]

/kind feature

[kerry-he]

Fixed, tags are now a separarate field and uses its own table.