esp_mg_net_if refactoring: split LWIP & ESP parts

esp_mg_net_if was a rat's nest of mongoose net_if and event manager
implementaions and the event loop task for the ESP8266 miot port.
From that, pieces that are related to LWIP support have been pulled out,
separated into net_if and event manager files, and event loop task
has been moved under the miot esp8266 dirrectory.

This is done to facilitate LWIP code reuse. This may not be the end of
it, but it's a start.

Note: custom retransmit logic has been removed for now. Can be
reintroduced later is necessary.

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