Use `required` keyword with non-nullable types for required DTO fields.

[ejball]

This could be a breaking change to older client code for the same API, so it might need to be opt-in.

[ejball]

Unfortunately, the entire HTTP mapping layer depends on the DTO having the new() generic constraint and all properties being nullable.

The new() generic constraint issue could be circumvented with Activator.CreateInstance<T>(), though not without hurting efficiency. Otherwise we'd need to overhaul the implementation to have every property value ready when the DTO is initialized.

Another difficult case is when a response supports multiple HTTP status codes. A normal body field can be required, but only if the normal status code is used. I suppose we could just skip required/non-nullable in that somewhat rare case.

One more difficulty is that we use the DTO as the JSON request/response body, even if some of the fields are set via path/query/header. Maybe we'd need to codegen an internal DTO with just the body fields.