Merge Spec and Status ARM types

[theunrepentantgeek]

Describe the current behavior

We are currently synthesizing separate ARM types for the Spec and Status of a resource. The spec ARM type used exclusively for resource PUTs and the status ARM type is used exclusively for resource GETs.

While the two ARM types have different shapes (each has properties the other lacks), ARM itself doesn't make a distinction and uses the same shape for both directions.

Describe the improvement

Merge the two ARM types into a single payload type, consistent with the way ARM works.

Additional context

Split out from #1588.

Has #3982 as a prerequisite.