feat: Implement vrf router with auto-allocating evpn_vni attribute

[stevekeay]

This creates a standalone plugin for 2026.1 to provide the vrf router behaviour we need, including auto-assignment of VNIs from a configured range.

Some of this overlaps work in progress upstream. I have intentionally mirrored the --evpn-vni API surface of the evpn plugin. We are not using any of the actual evpn plugin code but (in theory) once it gains feature parity, we should be able to switch over to it without affecting our users.

This got fairly big. I was thinking it might be cleaner to break it out into a separate package (that we would install separately via pip install in the container) but I wanted to solicit opinions from the team.

To use this:

• we can roll neutron back to stock 2026.1 (removing the stuff we cherry-picked)
• change the neutron.conf service_plugins removing evpn and adding understack_vni
• Add a [understack_vni] section like vni_ranges = 10000:19999