`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