Allow setting code system violation severity on UnknownCodeSystemWarningValidationSupport

[TipzCM]

Describe the bug
UnknownCodeSystemWarningValidationSupport does not throw warnings if set to do so.

Currently it's either "allow" or "not allow", and "allow" is non functional.

I was expecting that setting the validation policy config to GENERATE_WARNING would cause smile to return warnings when we provided an arbitrary system in a coding data type.

However, the following resource fails validation with the error Unknown Code System 'http://hl7.org/fhir/universal-tag'

To Reproduce

{
    "resourceType": "Patient",
    "meta": {
        "tag": [
            {
                "system": "http://hl7.org/fhir/universal-tag",
                "code": "golden"
            }
        ]
    }
}

Expected behavior
Allowing warning behaviour should be allowed

Environment (please complete the following information):

• HAPI FHIR Version 5.7.0-PRE2-SNAPSHOT