Validate required bindings from the base spec

[lmsurpre]

For code values with a required binding, we generate Code subtypes in the model and invalid values cannot be parsed.
However, for CodeableConcept values with required bindings in the base spec, we don't validate the concepts at all.

Now that we have valueset membership operational, I think we should start using this to validate valueset membership for required bindings in the base spec.

To reproduce:
Create or validate a resource that has an element of type CodeableConcept and a ValueSet binding of Required.
Note that the server happily accepts any value for this element, even if there is no coding from the required ValueSet.

This should fail validation:

{
  "resourceType": "Condition",
  "id": "example",
  "clinicalStatus": {
    "coding": [
      {
        "system": "http://example.com",
        "code": "fake-code"
      }
    ]
  },
  "subject": {
    "reference": "Patient/example"
  }
}

This should pass validation:

{
  "resourceType": "Condition",
  "id": "example",
  "clinicalStatus": {
    "coding": [
      {
        "system": "http://example.com",
        "code": "fake-code"
      },
      {
        "system": "http://terminology.hl7.org/CodeSystem/condition-clinical",
        "code": "active"
      }
    ]
  },
  "subject": {
    "reference": "Patient/example"
  }
}