[Validator] Add support for R5 valueAlternatives and mustHaveValue

[ewoutkramer]

For R5, ElementDefinition is extended with new properties influencing cardinality validation of a primitive, see http://build.fhir.org/elementdefinition.html#min-max

Describe the solution you'd like
To support this functionality, we need to add support for valueAlternatives and mustHaveValue, as mentioned in the referenced page for R5.