Rollup merge of #86434 - CDirkx:ipv6-benchmarking, r=joshtriplett

Rollup merge of #86434 - CDirkx:ipv6-benchmarking, r=joshtriplett

Add `Ipv6Addr::is_benchmarking`

This PR adds the unstable method `Ipv6Addr::is_benchmarking`. This method is added for parity with `Ipv4Addr::is_benchmarking`, and I intend to use it in a future rework of `Ipv6Addr::is_global` (edit: #86634) to more accurately follow the [IANA Special Address Registry](https://www.iana.org/assignments/iana-ipv6-special-registry/iana-ipv6-special-registry.xhtml) (like is done in `Ipv4Addr::is_global`).

With `Ipv6Addr::is_benchmarking` and `Ipv4Addr::is_benchmarking` now both existing, `IpAddr::is_benchmarking` is also added.