Investigate HAPI instance validation

[jingtang10]

Describe the issue to be researched
We are doing a lot of validation work on the questionnaire and questionnaire response. e.g. #926 and https://github.com/google/android-fhir/blob/master/datacapture/src/main/java/com/google/android/fhir/datacapture/validation/QuestionnaireResponseValidator.kt

But it's not clear to me if and how https://hapifhir.io/hapi-fhir/docs/validation/instance_validator.html could help with this process. Is the code we're writing simply dupilcating existing functionality in hapi instance validator? Can hapi instance validator correctly validate the rules in the SDC spec and in the FHIR questionnaire spec?

Describe the goal of the research
Find out the capablities of the hapi fhir instance validator. Compare with our validator. Provide comparison tables and analysis. Give recommendations of how we should proceed in terms of the validation code.

Describe the methodology
Test the hapi instance validator: https://hapifhir.io/hapi-fhir/docs/validation/instance_validator.html
Writing down the findings
Discuss with the implementers
Provide recommendations and create issues in this repo
Finally update the documentation in our wiki

Would you like to work on the issue?
no