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This commit adds a new `network.PPP` interface which works on any port that
has bare-metal lwIP, eg rp2, stm32, mimxrt.

It has been tested on stm32.  A board needs to enable
`MICROPY_PY_NETWORK_PPP_LWIP` and then it can use it as follows:

    import network

    ppp = network.PPP(uart)
    ppp.connect()

    while not ppp.isconnected():
        pass

    # use `socket` module as usual, etc

    ppp.disconnect()

Usually the application must first configure the cellular/etc UART link to
get it connected and in to PPP mode first (eg ATD*99#), before handing over
control to `network.PPP`.

The PPP interface automatically configures the UART IRQ callback to call
PPP.poll() on incoming data.
Signed-off-by: Damien George <damien@micropython.org>