annotate doesn't support specifying ref

[aguschin]

related to #25

Right now, annotate on remote repo can create a commit on top of default branch only:

$ gto annotate classifier \
		--repo $REPO \
		--path model.pkl \
		--description "Simple text classification model"
		--type model```

There should be a way to annotate in non-default branches, e.g.

$ gto annotate classifier \
		--repo $REPO \
		--branch try-large-dataset
		--path model.pkl \
		--description "Simple text classification model"
		--type model

Question: should we support creating a new branch with annotation out of existing ones? E.g.

$ gto annotate classifier \
		--repo $REPO \
                --rev try-large-dataset
		--branch new-branch
		--path model.pkl \
		--description "Simple text classification model"
		--type model