Add list and detail serializer patient external serializer

[GeekGawd]

Proposed Changes

• Add list and detaiil serialiser to patient_external.py
• Add test cases to test the changes.

Associated Issue

• Fixes patient_external.py in https://github.com/coronasafe/care_fe/issues/5492

Merge Checklist

• [x] Tests added/fixed
• [x] Linting Complete

Only PR's with test cases included and passing lint and test pipelines will be reviewed

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