[🚀 Feature Request] Relax request/reply types in the services

[echarles]

Problem

For now the @jupyterlab/services packages (https://github.com/jupyterlab/jupyterlab/tree/master/packages/services) is very strict in terms of API (as well for the methods and for the request/reply types.

This may sound logical as the services enforces norms, protocols in a secure ways, but it also begins in practice to create limitations as discussed on #11678 and #11285

Proposed Solution

We should discuss to find a middle ground that allow services consumer to extend the request/reply.

Additional context

So far, we have the following case where this would be useful

• Kernel Resource Usage: Add a usage request/reply on the Kernel Control channel #11285
• User service: Switching from user extension to user service #11678