ec_host_cmd: improve handling IN_PROGRESS commands

Add the ec_host_cmd_send_in_progress_continue function which allows
continuing execution of a handler, while the ec_host_cmd thread is not
blocked and new commands can be handled.

That means some long command handlers can executed in the background
while the Host Command subsys is not frozen.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>