Add support for BGPaaS

[wdesmedt]

PE-CE routing in the overlay.

[wdesmedt]

This can be done with Level 1 Infra intents but it's touching network_instance var at multiple places (groups, neighbors).
Might be an idea to have a Level 1.5 intent that describes a 'BGP Peer' as a single object, but as this is a node-specific intent (bgp peers have IP address info attached which is linked to the node) the benefits are less clear as with level-2 intents