[RFE] OVN BGP Agent EVPN Advertisement API Extension

[Lucas Alvares Gomes]

The work adding support for L3 VPN for OVN BGP Agent is now merged [0] and the only way to configure the VNI and Type field on a Logical Switch (aka Neutron Network) is by using the "ovn-nbctl" command to write the values directly to the OVN Northbound Database, for example:

    $ ovn-nbctl set logical-switch neutron-cd5d6fa7-3ed7-452b-8ce9-1490e2d377c8 external_ids:"neutron_bgpvpn\:vni"=100

    $ ovn-nbctl set logical-switch neutron-cd5d6fa7-3ed7-452b-8ce9-1490e2d377c8 external_ids:"neutron_bgpvpn\:type"=l3

This is not ideal as we do not want users to be changing databases directly. Therefore we need an API extension so these values can be populated safely.

[0] https://review.opendev.org/c/openstack/ovn-bgp-agent/+/906505

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/neutron-specs/+/920681