Fix BMD Bootloader to handle DFU_DETACH requests explicitly

[ALTracer]

Detailed description

• This is a minor feature with no new code.
• The existing problem is BMD bootloader ignoring dfu-util --detach requests. It still does reboot on dfu-util -s :leave.
• This PR solves it by adding an explicit request handler.

DFU runtime is unaffected, it already handles DFU_DETACH.
BMD bootloader still auto-reboots in Manifest phase when it has written a new firmware, this is not about that.
Tested on blackpill-f411ce with dfu-util -v and bmputil.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (see Building the firmware)
• It builds as BMDA (see Building the BMDA)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues