Adds value-ranking support to result ordering

[djperrefort]

This PR introduces a DRF ordering backend called PrecedenceOrderingBackend that extends the default ordering behavior with an optional _rank query parameter. Clients continue to sort with the standard _order parameter, but can now supply an explicit value ranking per field as _rank=field:value1,value2,value3. Rows are ordered by each value's position in the rank list with unranked values sorting last and the - prefix on _order reversing the result. A field named in _order without a matching _rank falls back to its natural column order. In keeping with current API standards, malformed _rank terms are silently ignored. Because the backend inherits from DRF's OrderingFilter, it reuses the existing field-validation and default-ordering logic, honors the ORDERING_PARAM setting, and self-documents both parameters in the generated OpenAPI schema. Full unit test coverage is included for parsing, ranking, ordering behavior, and schema generation.

[djperrefort]

This was a fun exercise, but is ultimately overengineered for the projects needs. Closing.