Implementation of ::isLinked() for ethernet drivers:
This function returns true when the ethernet chip is physically connected.

(edited) It currently works and has been tested with W5500 and ENC28J60, and it seems it is not implementable with W5100.
An additional ::isLinkDetectable() tells when ::isLinked() is not trivially implemented.

(edited)::connected() is software only and is left unmodified for backward compatibility is updated to check link status to solve compatibility with WiFi API.

Closes #8099