Contact endpoint includes External ID

The integrity of syncs with various school districts can be improved by storing the student's foreign key.

DB Dependency:
The current DYCF ID field needs to be repurposed and renamed. Further, this field may need a supporting "district ID" field as well. That should be evaluated before implementing.

Client App Requirement:
https://github.com/AmericaSCORESBayArea/External-Sync-Robot/issues/45

Use Case:
- Sync Robot requests a record for a student with a common name. Several results can be returned. None of them match the birthdate but several have no birthdate. The Sync Robot incorrectly assumes a new record needs to be created. Matching on the unique foreign Key helps reduce this error.