Making the 'gh jira <issue> --assignee <name>' work for when the user doesn't want to perform any transition & refactoring the ASSIGN_TO_ME action to make it simpler

Signed-off-by: Henrique Vicente <henriquevicente@gmail.com>
Update the assignee when updating transitions with --assignee

Signed-off-by: Henrique Vicente <henriquevicente@gmail.com>