rp2040/rp2350: Support multicast TTL socket option

[Daniiiil1]

Closes #9938.

The Raspberry Pi port uses lwIP's raw API rather than its socket API, so it cannot delegate this option to lwip_setsockopt() as the Espressif port does. Handle IPPROTO_IP / IP_MULTICAST_TTL for UDP sockets by updating the UDP PCB's multicast TTL directly.

The implementation also:

• returns EOPNOTSUPP for non-datagram sockets;
• returns EINVAL for a closed UDP PCB or an empty option value;
• follows lwIP's one-byte IP_MULTICAST_TTL representation.

Testing:

• pre-commit run --files ports/raspberrypi/common-hal/socketpool/Socket.c
• gmake -C ports/raspberrypi BOARD=raspberry_pi_pico_w -j4
• gmake -C ports/raspberrypi BOARD=raspberry_pi_pico2_w -j4
• inspected both generated objects to confirm the option path writes one byte to udp_pcb.mcast_ttl

Both builds produced firmware.uf2. I do not have a Pico W/Pico 2 W connected, so I have not performed an on-device multicast packet capture.

[tannewt]

@todbot want to test this? The issue from jepler says it is needed for your MicroOSC.