[WIP] XAP -- eXtensible Application Protocol

[tzarc]

Description

This PR is used to track the effort for the actual implementation of XAP, as per #11567.

Initial scoping document here: https://hackmd.io/hhjooQ6UQhWV3MvTlbQiqQ?both

Work in progress.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• XAP (née QMK API) discovery and scoping #11567

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[Drugantibus]

Amazing to see progress being made so fast! Please count me in for any testing