Add support for a configurable "fail fast" validation feature

[JohnTimm]

In production, it makes sense to fail fast when the FHIRValidator encounters an issue with an ERROR severity. A configurable "fail fast" validation feature would allow for development with standard behavior and production with a more performant option.

[prb112]

Created an Observation without issues
Created an Observation (same starting point without a validQuantity) and FailFast as false
Two Errors present
Created an Observation with same starting point and failFast as true
One Error Present (as expected)

    "resourceType": "OperationOutcome",
    "id": "c0-a8-56-14-d7cf56ff-323d-4b1b-881c-130604e5548f",
    "issue": [
        {
            "severity": "error",
            "code": "invariant",
            "details": {
                "text": "vs-3: If there is no a value a data absent reason must be present"
            },
            "expression": [
                "Observation.component[0]"
            ]
        },
        {
            "severity": "warning",