Pre-proposal: Debugger protocol - add a "complete" request next to the "evaluate" one

[martinRenou]

In jupyterlab/jupyterlab#17892 we started looking into implementing a proper repl console in debugging context.

This PR allows to easily explore the current scope when stopped at a breakpoint.

For now that PR supports:

• executing code using the current evaluate message of the debug protocol
• completing code using a custom Python code that is evaluated to evaluate the completion results.

Instead of hardcoding this completion code there, it should be the kernel's responsibility to implement a proper complete request. Similar to the kernel complete_request but for the debugger.

Ideally, we would go further and implement most messages:

• is_complete_request to check that the code is complete
• complete_request to auto complete
• history_request to browse the history