Allow Access Point static IPv4 on the raspberrypi port

[anecdata]

Like #7946 but for raspberrypi. The two PRs together should fully address #7931.

Draft for now... took a first stab at PicoW AP static IPv4. A couple of oddities...

1. Unlike espressif, AP needs to be started before the IPv4 address can be set:

Adafruit CircuitPython 8.1.0-beta.2-33-gcd41fb101 on 2023-05-14; Raspberry Pi Pico W with rp2040
>>> import wifi
>>> import ipaddress
>>> 
>>> ipv4 = ipaddress.IPv4Address("192.168.251.2")
>>> netmask = ipaddress.IPv4Address("255.255.255.0")
>>> gateway = ipaddress.IPv4Address("192.168.251.1")
>>> wifi.radio.set_ipv4_address_ap(ipv4=ipv4, netmask=netmask, gateway=gateway)
>>> wifi.radio.ipv4_address_ap
>>> 
>>> wifi.radio.start_ap("Bob", "YourUncle")
>>> wifi.radio.ipv4_address_ap
192.168.4.1
>>> 
>>> wifi.radio.set_ipv4_address_ap(ipv4=ipv4, netmask=netmask, gateway=gateway)
>>> wifi.radio.ipv4_address_ap
192.168.251.2

This means that there will be an interval where something could connect to the AP with the wrong IP range, and that address could conflict with another network.

2. The connecting station gets IP address 192.168.4.16 (expected 192.168.251.3), so something more needs to be done with the AP DHCP server.

[anecdata]

Turns out that lwip only provides the DHCP client. The DHCP server comes from shared/netutils, and it is inited only once, deep in the cyw43 driver when wifi is inited:
https://github.com/georgerobotics/cyw43-driver/blob/05d76fe0091f3a3383d5e310b604ddc7d9556ed2/src/cyw43_lwip.c#L236
I'm not sure if or how the DHCP server can be turned off and re-started, without a lot of side effects.

[anecdata]

Turns out that the netutils DHCP server can be turned off and back on again without obvious side effects.

AP:

Adafruit CircuitPython 8.1.0-beta.2-32-gcf124ab85 on 2023-05-14; Raspberry Pi Pico W with rp2040
>>> import wifi, ipaddress
>>> 
>>> ipv4 = ipaddress.IPv4Address("192.168.251.2")
>>> netmask = ipaddress.IPv4Address("255.255.255.0")
>>> gateway = ipaddress.IPv4Address("192.168.251.1")
>>> wifi.radio.start_ap("Bob", "YourUncle")
>>> wifi.radio.set_ipv4_address_ap(ipv4=ipv4, netmask=netmask, gateway=gateway)
>>> wifi.radio.ipv4_address_ap
192.168.251.2

Station (separate device):

>>> import wifi
>>> wifi.radio.ipv4_address
>>> 
>>> wifi.radio.connect("Bob", "YourUncle")
>>> wifi.radio.ipv4_address
192.168.251.16

Tested by connecting to the AP from another CircuitPython device and from a desktop computer. Also ran a TCP echo server on the static IPv4 PicoW AP, and sent packets to it from a TCP client on the connected CircuitPython station:

Output from TCP server on the AP...

code.py output:
Create TCP Server socket ('192.168.251.2', 5000)
Listening
Accepting connections
Accepted from ('192.168.251.16', 57117)
Received bytearray(b'Hello, world') 12 bytes
Sent bytearray(b'Hello, world') 12 bytes
# ...etc.

Output from TCP client on the station...

code.py output:
Self IP 192.168.251.16
Server ping 192.168.251.2 0.0 ms
Create TCP Client Socket
Connecting
Sent 12 bytes
Received 12 bytes bytearray(b'Hello, world')
# ...etc.

I don't know why the station IP address jumps to .16 (espressif increments connected station IPv4 adddresses by one from the AP IPv4), but it does this in the default DHCP case also (without static IPv4).

I'd appreciate careful review, this is my first dive into the depths of the PicoW internals.

[bill88t]

I have only fumbled a little bit with lwip (back when I did AP), but looks good to me.

[tannewt]

I'm fine merging as-is. I don't know a ton about the LWIP internals either but if it works, then it should be ok.

[anecdata]

The changes are isolated to the affected API, so there should be minimal risk to code that is not using wifi.radio.set_ipv4_address_ap (or for some other reason trying to stop DHCP and restart it).

I realized I didn't put in the #if CYW43_NETUTILS conditional that is in the georgerobotics cyw43 driver. Not sure if that's expected to be replicated at this level. But the #if LWIP_DHCP conditional doesn't appear in our code either.