Extend PCAL6416A driver to implement `digital::InputPin` and `digital::OutputPin` from `embedded-hal`

[madeleyneVaca]

It would be nice to have an additional abstraction on top of this driver using digital::InputPin and digital::OutputPin from embedded-hal

So instead of doing ioexp_device.config_port_0().write_async(|w| w.set_c_0_7(true)),
we can do

   let pin = ioexp_device.get_pin(0, 7);
   pin.set_high();

Originally posted by @jerrysxie in #1 (comment)

[Wassasin]

Recently the digital traits have been added in embedded-hal-async too @ rust-embedded/embedded-hal#649. We have to wait for a new release of the crate though.