Expose invariant ids in operationoutcome-message-id extension for failed invariants

[alexzautke]

In case any invariant fails during validation, make sure that in the end we add the https://build.fhir.org/ig/HL7/fhir-extensions/StructureDefinition-operationoutcome-message-id.html extension to the OperationOutcome returned as the result. The value should contain the id of the failed invariant, e.g. "ele-1".

[ewoutkramer]

We currently have an internal string IssueSource on the IssueAssertion (which makes up the ResultReport of which OperationOutcome is derived). If we'd change that to public IAssertion IssueSource, we could access any configuration information of the validators, so we can more generally supply any info from the schema (and structuredefinition) into the OperationOutcome.

Of course, we'd also need to add an overload to public static OperationOutcome ToOperationOutcome(this ResultReport result) with a callback of type Action<IssueComponent,IssueAssertion>, which would allow the callback to use the info available in the IssueAssertion (DefinitionPath, IssueSource) to enrich the IssueComponent before it is added to the operationoutcome.