Adds `getRoles` function.

In our use of node relations, we take advantage of dynamic roles to define a permissions schema where the `role` is really a Team id and all the `subject`s are members of that team. It would be useful for us to be able to access the in-memory definition of what `roles` map to a `verb`. For example, we may want to ask what team can execute an action, rather than asking who the subjects are. 

This PR adds what seems like a pretty simply addition of a `getRoles` function, which returns a filtered list of all the roles in a context that map to a specified verb.