Add BelongsToEntity validator constraint

Similar to RowExists, this constraint would ensure that a row matching an id exists in the database.  Additionally, it will check that the entity found has a foreign key column whose id is equal to the id of the entity being edited (validation ~~will~~ may fail for POSTs, since the resource in question can't belong to an entity that doesn't exist yet).  This will require that the validator has access to the entity being edited, so changes to AbstractArrayValidator will be necessary.

### Acceptance Criteria
1. None

### Tasks
- None

### Additional Notes
- None