Augment model with BGP neighbor update source

[Fredi-raspall]

This change is needed for preview.

The conversion from gRPC in dataplane is not correct as it uses the neighbor address to set the update source address towards the neighbor. In order to fix this, the gRPC protobuf schema should allow setting this. This patch augments the protobuf schema with such a field.

The issue explains why we were seeing things like:

2025/05/20 12:29:52 BGP: [VZ06J-9VWXE][EC 100663299] can't bind socket for 7.0.0.2 : Cannot assign requested address

... because the config generated for FRR was:

 neighbor 7.0.0.2 update-source 7.0.0.2

which is incorrect.

In dataplane, this is temporarily fixed by githedgehog/dataplane#498

[Frostman]

What values would you expect to be passed? Loopback with its ip?

[Fredi-raspall]

What values would you expect to be passed? Loopback with its ip?

• FRR can admit an IP address or an interface name.
• I don't think we should speculate here (autogenerate) neither with ips nor interfaces.
• Interface may not necessarily be the loopback. If we establish bgp per link, then it could be (ought to be) the interface name (or address). Given that, even with a bgp-only implementation, we may support distinct types of setups (some with multiple loopback interfaces or addresses), I think that the best option is to expose this.