feat(transport): expose server implementation on transport

[NathanBaulch]

Description (what this PR does / why we need it):

Expose the server implementation on Transporter, similar to what's available in GRPC interceptors. This gives middlewares the ability to make decisions or delegate functionality to the server/handlers that do the work.

Example middleware that delegates authentication checks to the server handler:

func Server() middleware.Middleware {
	return func(handler middleware.Handler) middleware.Handler {
		return func(ctx context.Context, req any) (any, error) {
			if tr, ok := transport.FromServerContext(ctx); !ok {
				return nil, errors.Unauthorized()
			} else if auth, ok := tr.Server().(Authenticator); !ok {
				return nil, errors.Unauthorized()
			} else if ctx, err := auth.Authenticate(ctx, tr); err != nil {
				return nil, err
			} else {
				return handler(ctx, req)
			}
		}
	}
}

This is an additive change that requires HTTP servers to be regenerated since they need to call http.SetServer alongside the existing http.SetOperation.

[shenqidebaozi]

This change looks strange, I don't know why it's done this way

[NathanBaulch]

I'm more than happy to answer any questions. My approach exactly mirrors the way Transport.Operation (aka grpc FullMethod) is intercepted and exposed to middlewares.