Ensure fhir path is used to determine value path

[juliangruendner]

Ensure fhir path is used to determine value path instead of making it dependent on ResourceType.

1. Add fhirPath to "MappingContext" class
2. Change all classes in structured_query where "Expression" is created in toCql function to use fhirPath from (in step 1) adjusted MappingContext

Step 2 would for example change the following line of code:
https://github.com/num-codex/codex-sq2cql/blob/main/src/main/java/de/numcodex/sq2cql/model/structured_query/NumericCriterion.java#L84

where instead of value it would use the respective fhir path from the MappingContext