Allow different model property definitions per visibility

#2926 is a special case of a more general scenario - the ability to have fine-grained control over how a property appears in different visibilities. This might include optionality (e.g. optional on input, but normalized to a default value by service and always sent in output), or decorator metadata (e.g. a string with a free-form input format and more constrained output format).

If you could somehow declare different properties per visibility, you would have a lot of flexibility. You might consider something like declaring separate properties and using `@encodedName` to make them all the same name on the wire, except for two issues: `@encodedName` checks for conflicting declared member names and does not consider visibility, and also in this scenario the user doesn't care about the protocol but is modelling at the conceptual layer.