[OVN][OVN-BGP-AGENT] Expose subnet CIDR information on NB DB

[Lucas Alvares Gomes]

In ML2/OVN, the OVN equivalent of a Neutron Subnet is a DHCP_Option entry in the OVN Northbound database but, when DHCP is disabled for a Neutron Subnet no entry is created.

Projects like ovn-bgp-agent does not talk directly to Neutron, instead it gets information from the OVN Northbound database that is populated by ML2/OVN.

For the multi-cloud connect with BGP work, the ovn-bgp-agent needs to know the CIDR of the provider network but, if DHCP is disabled we do not have that information at handy as the DHCP_Option entry is not present.

This LP is addressing that problem and proposing storing the subnet's CIDRs in the Logical_Switch table (the Neutron Network equivalent in OVN) external_ids column. With that, projects such ovn-bgp-agent can consume this information while doing it's configuration.