[Core] IPv6 Support

[Basasuya]

Description

We are planning to extend Ray to support IPv6-only clusters. There have been a couple previous efforts and issues reported; this issue plans to consolidate all references:

• [core] ipv6 support #6967
• Ray IPv6 support #44252
• [cluster] bringing up cluster fails: socket.gethostbyname("localhost") - socket.gaierror: [Errno -2] Name or service not known #43416
• Java : Fail to parse IPv6 address and port from bootstrap address in GcsClientOptions #41966

Use case

Details and Design
https://docs.google.com/document/d/1MavJNSYxj-AJNeAT7ETtB2qgOpd5pZQRgTahS2moWgc/edit?usp=sharing

[codope]

@Basasuya Thanks for sharing the design doc. Before we provide detailed feedback and clarifying questions, could you confirm a few things:

1. Is this design focused solely on enabling IPv6 within Kubernetes hostNetwork mode, or is future extensibility to CNI-based pods also expected?
2. Does the design consider dual-stack (IPv4 + IPv6) environments in its goals, or is it IPv6 only? Ideally, we should aim for dual stack.
3. Are there specific failure modes or bugs you've already seen (besides port binding / socket issues) that the PR addresses?

[Basasuya]

@codope

1. it would also works on other cni-based pods
2. aim for dual stack. but all ray inner connection would prefer ipv6
3. No. We've used this implementation internally in our company for some time. If there are any issues, I think they can be solved.