Add a testing-only option that turns off IPv4 extends
To test that relays perform IPv6 extends correctly, we should turn off all IPv4 extends in a test network.
The option should be called ExtendAllowIPv4Addresses, and it should be documented and implemented like ExtendAllowIPv6Addresses in #33818 (moved).
We might also need an ExtendByIPv4ORPort option, similar to ExtendByIPv6ORPort.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information