No support for MPI search

[szymonp-kainos]

Due to https://www.hl7.org/fhir/patient.html section 5.1.8 use of search query _query=mpi should be possible and return scored results.

Fhirbase for query

SELECT fhir_search('{"resourceType": "Patient", "queryString": "_query=mpi&name=smith"}');

returns

ERROR:  Error: No parser for special - query
DETAIL:  plv8_init() LINE 4011:           throw new Error("No parser for special - " + key);

and for

SELECT fhir_search('{"resourceType": "Patient", "queryString": "query=mpi&name=smith"}');

returns

ERROR:  Error: Not found SearchParameter {"base":"Patient","name":"query"}
DETAIL:  plv8_init() LINE 3779:       throw new Error("Not found " + rt + " " + (JSON.stringify(query)));