Add client connect cancel

[gkoh]

I've found a need to allow a user to cancel an active client connection attempt. For now, I've just direct called ble_gap_conn_cancel() which does the job and causes a NimBLEClient::connect() to abort.

It would be nice, but not necessary, to add something like NimBLEClient::cancel() or similar so abstractions are preserved.

[h2zero]

Hi @gkoh, that's a good idea and would be very easy to implement.