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Enable use of wired Ethernet modules as first-class LWIP citizens.  All
networking classes like MDNS, WebServer, HTTPClient, WiFiClient, and OTA
can use a wired Ethernet adapter just like built-in WiFi.

Two examples updated to show proper use.

Uses the Async Context support built into the Pico SDK.  When running on the
Pico  it will use the CYW43 async instance.

Uses modified wired Ethernet drivers, thanks Nicholas Humfrey!

Note, the classic, non-LWIP integrated `Ethernet` and related libraries
should still work fine (but not be able to use WebServer/HTTPS/etc.)

Fixes #775