De-generify client and endpoint codegen?

[sfackler]

We currently rely on generic type parameters to represent a few things in the codegen for clients and servers:

• The underlying "raw" HTTP client is a type parameter on the generated client type.
• For streaming client requests and streaming endpoint responses, the body writer type.
• For streaming client responses and streaming endpoint requests, the body type.

This has a few disadvantages:

• For clients, the name gets a bit stutter-steppy: SomeServiceClient<Client>.
• For endpoints, the parameter bounds will cause methods on the endpoint to not be callable with the standard self.some_method() convention. If there are any endpoints with a streaming response, there will be a type parameter on the trait that's only used as a bound on the associated response type. When making a method call, that parameter won't be specifiable so you either need to fall back to the UFCS syntax: <Self as SomeService>::some_method(self) or add the type parameter to your implementing type via a PhantomData.
• For streaming requests and responses in both clients and endpoints, the type you get back is oddly underspecified. The standard conversion traits will have Stream<Bytes> or Iterator<Bytes> requirements, and the actual underlying types in witchcraft-server and conjure-runtime have some additional methods to handle trailers.

We could de-generify all of this logic:

• Add AsyncBody, and AsyncBodyWriter traits to conjure-http. These'll probably need to extend AsyncRead/AsyncWrite and Stream<Bytes>/Sink<Bytes>, and have the extra trailer methods.
• The AsyncClient trait loses its BodyWriter and ResponseBody associated types in favor of Pin<Box<dyn AsyncBodyWriter>> and Pin<Box<dyn AsyncBody>> trait objects.
• Generated clients now hold a Box<dyn Client> instead of being parameterized over the client type.
• Generated endpoints now work with Pin<Box<dyn AsyncBodyWriter>> and Pin<Box<dyn AsyncBody>> trait objects instead of being parameterized.

The equivalent changes would be made to the blocking and local async APIs.