So far the maxmium encryption key size has been hardcoded to 16 bytes in
'send_pairing_request_command()' and 'send_pairing_response_comman()'. By
making this configurable via the PairingDelegate, one can test how devices
respond to smaller encryption key sizes. Default remains 16 bytes.
Even through the previous implementation was correct:
- Always call `delegate.confirm()` for `just-works` pairing, but with
`auto` parameter set to `True`.
- Trust the controller and do not double check the devices IO
capabilities.
