Refactor more host code (programmable button & digitizer)

[fauxpark]

Description

Removing programmable button from host_driver_t (to match digitizer and joystick) and hoisting report assembly out of USB drivers.

Eventual goal is to replace the host_driver_t's send_keyboard, send_mouse and send_extra with a single send_report which takes endpoint number, pointer to report, and size.

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

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).