GONRG-1213: Add source file validation

src/dags/libs/exceptions.py

src/dags/libs/file_source_check.py

src/dags/libs/process_manifest_r3.py

src/plugins/operators/process_manifest_r3.py

tests/plugin-unit-tests/test_file_source_check.py

tests/plugin-unit-tests/test_manifest_processor_r3.py

tests/plugin-unit-tests/test_operators_r3.py