[3.5] Backport: netutil: consistently format ipv6 addresses

[Kidsan]

This is a backport of #15153

This formats ipv6 addresses to ensure they can be compared safely

Signed-off-by: kidsan 8798449+Kidsan@users.noreply.github.com

[ahrtr]

LGTM

[ahrtr]

Please add a changelog item in https://github.com/etcd-io/etcd/blob/main/CHANGELOG/CHANGELOG-3.5.md

[Kidsan]

@ahrtr I hope I did that right: #15190

[ahrtr]

Leave it to @serathius to merge this PR since he has some comment in #15153 (comment).

Per my comment #15153 (comment), I think it's OK to merge this PR for now and add whatever additional IPv6 related test case in separate PRs.

[serathius]

Just a note, we shouldn't do out of order backports. This fix is already in v3.4, forcing me to merge it as it would break clear git history. Backports for v3.4 should only take from PRs already backported to v3.5 and v3.5 should only take from fixes on main branch (assuming that issue is present on all releases).

Next time let's agree on backport plan before we start merging.

[ahrtr]

Backports for v3.4 should only take from PRs already backported to v3.5

I am afraid I can't agree this. 3.4 and 3.5 have huge differences, not every bug fix applies both branches. Backporting 3.4 should be independent with 3.5. The only agreement should be that both 3.4 and 3.5 should be backporting from main.