Implement ethernet packet API for AsyncClient

[ZiggVert]

The WincWifi struct has the following APIs, which are missing on the AsyncClient struct;

pub fn read_ethernet_packet(&mut self, buffer: Option<&mut [u8]>, timeout: Option<u32>) -> Result<usize, StackError>;
pub fn send_ethernet_packet(&mut self, net_pkt: &[u8]) -> Result<(), StackError>;

As an aside, why is buffer an Option?, and could timeout instead be an Option<Duration> or renamed to timeout_ms.

[kaidokert]

Thanks for sending the issue ! Note that AsyncClient is very new and still being worked on - it's far from complete, TCP implementation only landed last week.

The Option<> for recv buffer is an artifact of adapting to smoltcp interface - it expects an internal buffer. Looking at it now, perhaps this method should have been split in two. ( read_ and poll_ )

Good call on timeout: yes, this should be Option<core::Duration>