Consider moving StudentProgramAssociation and Service and ParticipationStatus to New Associations

As the Student Program model grows, the Service common type is becoming more complex. With the addition of a new common type ParticipationStatus, there is also the issue of multiple BeginDate/EndDate elements on the StudentProgramAssociation, Service, and ParticipationStatus.

To address these issues, we should consider moving ProgramService and ParticipationStatus into new associations.

We could also change the key to StudentProgramAssociation to not include BeginDate (may want to add SchoolYear as a replacement).

These would be breaking changes to the model and so should be considered for the next major release.