Add referred facility to discharge serializer

[Ashesh3]

Proposed Changes

• Add referred_to and referred_to_external to PatientConsultationDischargeSerializer to allow it to be set from the discharge form at the frontend.

Associated PR

• Frontend: added referred to facility field in discharge form care_fe#5730

@coronasafe/care-backend-maintainers @coronasafe/care-backend-admins

[sainak]

@Ashesh3 a test is failing, also add a test for when referred_to is given and when both referred_to and referred_to_external are given

[Ashesh3]

@Ashesh3 a test is failing, also add a test for when referred_to is given and when both referred_to and referred_to_external are given

Fixed and added more tests to cover referred_to specifically.