This replicates an issue we're seeing in the OpenAPIv3 generator, where
if two request bodies are structurally similar (i.e. same primitive
types), but not identical (in this example both have a single string
attribute with the same name, but with different enum validations), the
request body for those two methods points at the same type schema.

There's a (previously-failing) test, where:
- two types which are 'similar' (both have a string-type attr called
  `my_attr`), but not identical (they are different enums)
- two different services with the same method, each referencing
  different types

The resulting OpenAPIv3 schema generates a single
`MethodEnumRequestBody` type, and uses it for the `requestBody` type of
both methods.

This is incorrect, since the two methods use a different enum.

The fix here is to update the `hashAttribute` method to hash the
`.Validation` of the attribute, if it is set 🎉