[linux] Add mechanism to use pre-configured IP interfaces

[turon]

Problem

WiFiConnectivityMgr is designed to completely control and provision a device's WiFi network interface.
The linux target should be able to use pre-configured / pre-existing IP interfaces such as Ethernet or an WiFi connection that has been already configured outside of the DeviceLayer.

Proposed Solution

<suggested fix, suggested enhancement>

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[turon]

#1975 begins to implement ConnectivityManagerImpl::_HaveIPv4InternetConnectivity for WiFi.

One path is to simply call InetLayer UDPEndpoint APIs directly, just assume connectivity exists.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[stale]

This stale issue has been automatically closed. Thank you for your contributions.