Refactor emrapi module's DiagnosisService to use new functionality

Refactor emrapi module's DiagnosisService to use this new functionality under the hood (e.g. either delegating to openmrs-core, or using the current emrapi approach, depending on the openmrs-core version)

The source of the module can be found at https://github.com/openmrs/openmrs-module-emrapi

Now create a DiagnosisServiceImpl class in the new api-2.2 sub module that Ola has created.
This class should implement the org.openmrs.module.emrapi.diagnosis.DiagnosisService interface and delegate these calls to the openmrs-core DiagnosisService implementation.