Support cqf expressions during validation

[mmsmits]

US-core 5 contains a profile (http://hl7.org/fhir/uv/sdc/StructureDefinition/sdc-question-library) which has an ElementDefinition like this:

      <path value="Demographics.dob"/>
      <label value="What is this person&#39;s date of birth?"/>
      <definition value="n/a"/>
      <min value="0"/>
      <max value="1"/>
      <base>
        <path value="Demographics.dob"/>
        <min value="0"/>
        <max value="1"/>
      </base>
      <type>
        <code value="date"/>
      </type>
      <minValueDate value="1900-01-01"/>
      <maxValueDate>
        <extension url="http://hl7.org/fhir/StructureDefinition/cqf-expression">
          <valueExpression>
            <language value="text/fhirpath"/>
            <expression value="now() as date + 1 day"/>
          </valueExpression>
        </extension>
      </maxValueDate>
    </element> 

I would be nice if we support the cqf extension for validation. This could be done in the schemabuilder