Extends the network plumbing to support per-interface IPv6 address
configuration.
The `Interface` protocol supports `ipv6Address` and `ipv6Gateway`.
The agent's networking RPCs carry per-family fields via new
`InterfaceAddress`, `LinkRoute`, and `DefaultRoute` types in
`ContainerizationExtras`.
`NetlinkSession` adds IPv6 methods for address and route operations.
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Co-authored-by: michael_crosby <michael_crosby@apple.com>
Co-authored-by: Michael Crosby <crosbymichael@gmail.com>
Yibo Zhuang