feature request: async support for FhirPath resolve()

[darena-antonw]

Is your feature request related to a problem? Please describe.

I would like the fhir path evaluation to resolve elements from a database / http call.

This is currently possible with: FhirEvaluationContext.ElementResolver

However the caveat is that I would have to introduce blocking in to my application, as the delegate can not be async.

Describe the solution you'd like

• new property AsyncElementResolver on FhirEvaluationContext of type Func<string,Task<ITypedElement>>
• new method Compile(string fhirPath, bool async) that evaluates to CompiledAsyncExpression
• new delegate type public delegate Task<IEnumerable<ITypedElement>> CompiledAsyncExpression(ITypedElement root, EvaluationContext ctx);

so in the end I can do the following:

var expression = compiler.Compile("entry.resource.ofType(MedicationRequest).medication.resolve().id", true);
var result = await expression(
    bundle,
    new FhirEvaluationContext
    {
        AsyncElementResolver = async (id) =>
        {
            // call to db or http
            return ...
        }
    }
);

Describe alternatives you've considered

it's possible with blocking via .GetAwaiter().GetResult()

Additional context

this was discovered during connectathon cds hooks testing.