Add option to configure debug probe speed

[sdonnan]

By default probe-rs uses a hardcoded debug adapter speed. For adapters that cannot meet that exact speed they may fall back to a slower one rather than use an internal sane default. This can slow down operation on some adapters (like mculink).

This change adds and optional command line flag to provide a speed setting.

An example of the impact this setting has:

~/w/h/t/release (feature/set-probe-speed)> time ./humility flash -F
humility: attached to archive
humility: attaching with chip set to "STM32H753ZITx"
humility: attached via CMSIS-DAP
humility: archive appears to be already flashed; forcing re-flash
humility: flashing done

________________________________________________________
Executed in   48.68 secs

~/w/h/t/release (feature/set-probe-speed)> time ./humility --speed 8000 flash -F
humility: attached to archive
humility: attaching with chip set to "STM32H753ZITx"
humility: attached via CMSIS-DAP
humility: archive appears to be already flashed; forcing re-flash
humility: flashing done

________________________________________________________
Executed in   34.44 secs

Tested with stlinkv3, mculink, and another generic daplink device.

[labbott]

This looks super helpful, thanks for digging into this