UTP12 — Attribute CodingRule::coding should be typed by ValueSpecification

The specification states that coding rules should be defined outside of the utp model and just being referred from the utp model.

In some situations, it can might be required to include a precise coding and decoding of values already in the model.

Therefore, the coding attribute should be typed by ValueSpecification, which would allow both merely naming the coding rule that should be used by an external test execution environment (LiteralString) and a formal representation of the coding/decoding mechanism itself (Expression/InstanceValue/OpaqueExpression)