Move 'Response' template objects somewhere

Problem/Motivation

The objects are not core to the operation of Symfony Messenger, and are simply conveniences.

We should be able to provide good examples for creating a message, message handler, and appropriate transport mapping to prove these convenience objects are not required.

Steps to reproduce

Proposed resolution

Remove them entirely, or
Move them to their own project
Provide good documentation and sample implementations in our test framework.

Remaining tasks

User interface changes

API changes

Data model changes