Feature Request: Add TTL mangling support (iptables-like)

[yamid1981]

It would be very useful to have a command similar to Linux iptables: iptables -t mangle -I POSTROUTING -j TTL --ttl-set 65

Use case: some ISPs restrict tethering by checking TTL values. Being able to set a fixed TTL (e.g. 65) on outgoing packets would allow ESP32 NAT Router to be used in more scenarios.

Implementation idea: modify the IP header in the NAT forwarding function to overwrite the TTL field with a user-defined value.