extmod: Add generic network.PPP to work with lwIP

[dpgeorge]

This PR 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()

Notes:

• lwIP has been updated to the latest stable version as part of this PR.
• 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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.39%. Comparing base (c3301da) to head (5e3dff1).
Report is 19 commits behind head on master.

❗ Current head 5e3dff1 differs from pull request most recent head dad02e5

Please upload reports for the commit dad02e5 to get more accurate results.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #14461   +/-   ##
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  Coverage   98.39%   98.39%           
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  Files         161      161           
  Lines       21204    21204           
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  Hits        20864    20864           
  Misses        340      340           

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[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS

[robert-hh]

That PR looks good, supporting the use of e.g. LTE modems or AT-Type WiFi interfaces.
Just a note: As you know RP2 and MIMXRT do not have yet the uart.irq method. That's done in PR #14041.

[dpgeorge]

Thanks @robert-hh for the review.

Just a note: As you know RP2 and MIMXRT do not have yet the uart.irq method.

Yes, thank you for that. That PR will definitely be needed!