feat: add manytoone approvers functionality

[sanchegm]

Description

• Create ForeignKey (ManytoOne) relationship for Affiliation and Approver based on docs: https://docs.djangoproject.com/en/5.0/ref/models/fields/#django.db.models.ForeignKey and https://docs.djangoproject.com/en/5.0/topics/db/examples/many_to_one/
• This relationship is ideal since we need one Affiliation to many Approvers.
• Create nested writable serializer for Approver relationship based on docs: https://www.django-rest-framework.org/api-guide/relations/#writable-nested-serializers Refactored tests to accommodate the above changes.

Issue Ticket Number and Link
 Related to Issue https://github.com/ClinGen/stanford-affils/issues/84

Checklist
Remove any options that are not applicable
 [X ] I have reviewed the how-to guide and the contributing file.
 [X] I have run required code checks and resolved any blockers.
 [X] I have created the necessary tests to show my changes are effective and work as intended.
 [X] I have thoroughly commented my code, especially in more complex areas.
 [X] I have updated any necessary documentation.

Screenshots / Recordings