fix: Add subnet ipv4/ipv6 to network schema

[TheRealFalcon]

Proposed Commit Message

fix: Add subnet ipv4/ipv6  to network schema

These are used by our openstack network_data.json parsing code and
get used by the sysconfig renderer.

Fixes GH-4911

Additional Context

#4911
and

cloud-init/cloudinit/net/sysconfig.py

Lines 623 to 636 in 70581bb

	is_ipv6 = subnet.get("ipv6") or is_ipv6_address(
	route["gateway"]
	)
	# Any dynamic configuration method, slaac, dhcpv6-stateful/
	# stateless should get router information from router RA's.
	if (
	_is_default_route(route)
	and subnet_type not in IPV6_DYNAMIC_TYPES
	):
	if (
	subnet.get("ipv4") and route_cfg.has_set_default_ipv4
	) or (
	subnet.get("ipv6") and route_cfg.has_set_default_ipv6

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[holmanb]

I'm almost tempted to ask if we can reorder that config change to after validation rather than supporting this as a user-facing config option for one (not used anymore?) renderer, but I haven't looked to see how difficult that would be.

Either way, looks good to me!