feat: add DO_IP_ADDR_FAMILIES env var for node address family control

[lohitkolluri]

Summary

Adds the DO_IP_ADDR_FAMILIES environment variable to control which IP address families are included in node status. This addresses the remaining ask from issue #555 — the basic IPv6 support was already merged in #898 but without the opt-in/opt-out mechanism requested by the maintainer.

Changes

• New env var: DO_IP_ADDR_FAMILIES accepts ipv4, ipv6, or ipv4,ipv6 (comma-separated)
• Defaults: When unset, all available addresses are included (backward compatible)
• Refactored: nodeAddresses now dispatches to a family-aware discoverAddress helper
• Validation: Invalid family values are rejected at startup with a clear error
• Tests: 7 new test cases covering all family combinations
• Docs: Updated both README.md and node examples README

Root Cause

The stale PR #561 implemented the env var approach but was never merged. Since then, basic IPv6 support was merged unconditionally in PR #898. This PR adds the env var gating on top of the current codebase, allowing operators to restrict to specific families.

Testing

• make ci passes (go vet, gofmt, unit tests)
• All existing tests pass unchanged — backward compatible by default
• New tests verify: default (no env), ipv4-only, ipv6-only, dual-stack, and invalid values

Risks

• Low — default behavior is identical to current (all families included)
• Setting DO_IP_ADDR_FAMILIES=ipv4 restricts to IPv4 only, which is the key new capability for dual-stack clusters that need to opt out of IPv6 node addresses

Closes #555
Signed-off-by: Lohit Kolluri lohitkolluri@gmail.com

[lohitkolluri]

Hello team just following up on this.