Introduce Branch to Registry Client and Version Control Information

For registry clients that support branching, there should be a way to get a list of branches, as well as pass in the branch on all the calls for saving/retrieving flows. Eventually this should be selectable from the UI on the screens where you choose Registry/Bucket/Flow/Version.

Any clients that don't support branching can just return a single hard-coded branch in the getBranches method, and can ignore the branch that is passed in on api calls.