Lightweight tags and tag namespaces

Lightweight tags are created with simple `git tag TAGNAME` and doesn't contain information about creator, date, etc. GTO doesn't create them, but user can. They can register versions or promote something.

Now they're ignored by GTO, although they still can trigger CI and though they look pretty similar to annotated ones for unexperienced user. We need to support them I guess.

I would suggest reading and interpreting them by default, but supporting some flag `--annotated` to filter them out.
Downside of this is that with lightweight tags you don't know date of promotion, author, etc, so `gto history` and `gto show myartifact` will have some fields blank.