Add support for $validate-code

**User story**
I want to validate the snomed code in .net core, how can i use "$validate-code" command in .net core for Azure API for FHIR

https://docs.microsoft.com/en-us/answers/questions/601649/index.html

**Acceptance criteria**
Can use $validate-code

No files, tests, or entry points are named. Start by reviewing the linked Microsoft Q&A and the FHIR $validate-code requirements for SNOMED codes in Azure API for FHIR. Done means the service supports the operation and the acceptance criterion, “Can use $validate-code,” is demonstrably met.

Written by the indexing model from the issue text.