Enable api.VolunteerAssignment.get to filter on assignee.

Out of expediency, we introduced some fairly inefficient client-side code to retrieve volunteer assignments related to a specific contact and a specific project. See linked commit.

As a result, we're pulling a list of all activities assigned to the contact and then filtering them via a chained API. It would be better to modify api.VolunteerAssignment.get to take parameters related to table civicrm_activity_contact so that we can filter on assignee.