Support a group of responses having different types across status codes, emitting an enum

[requiel20]

Closes #344. Also fixes the reports in #693, #950 and #1155.

A group carrying more than one type now generates an enum with a variant
per status code. The enums live in a response module, one submodule per operation,
so an operation that varies both halves reads as response::get_ready::Success
and response::get_ready::Error.

A group of one type still names that type directly, so no existing golden file changes
and no generated crate anywhere is affected.

Generated code:

  pub mod response {
      pub mod get_ready {
          pub enum Error {
              Status500(types::Err),
              Status503(types::Readiness),
          }
      }
  }

  pub async fn get_ready<'a>(
      &'a self,
  ) -> Result<ResponseValue<types::Readiness>, Error<response::get_ready::Error>>

Usage:

  use progenitor_client::Error;

  match client.get_ready().await {
      Ok(ready) => {
          let readiness: types::Readiness = ready.into_inner();
          println!("ready: {}", readiness.ok);
      }
      Err(Error::ErrorResponse(response)) => {
          let status = response.status();
          match response.into_inner() {
              response::get_ready::Error::Status503(readiness) => {
                  println!("not ready ({status}): ok={}", readiness.ok)
              }
              response::get_ready::Error::Status500(err) => {
                  println!("broke ({status}): {}", err.message)
              }
          }
      }
      Err(other) => println!("transport or unexpected status: {other}"),
  }

This PR supersedes #857, by working on its review notes:

• the generated types are in their own module, in the response::operation_id::Success
  shape suggested there;
• errors are covered as well as successes;
• the new concept lives inside extract_responses rather than beside it;
• there is a test whose generated output is compiled, not only compared.

The last of those needs #1421 first: progenitor-impl/tests/output has not compiled for
months, so on current main the new golden file is checked as text only. This change does
not depend on that one however, strictly speaking.