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 legacy/trac#33818 (moved).

We might also need an ExtendByIPv4ORPort option, similar to ExtendByIPv6ORPort.

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