Fetch interface list from `holod`

[frederic-loui]

For now when we configure holo via holo-cli, interface is entered in a free form.

It would be ideal to be able to configure an interface from holo-cli :

1. From a list of available physical interfaces

2. From a list of interface category of interface: te-tunnel, tunnel, bundle, hairpin, etc

3. Restrict sub-interface creation with . or unit notation form to:
   3.1. Listed physical interface in 1
   3.2. Or logical interface listed in 2

This will enforce configuration consistency.

For 1, it means that holo-cli should be able to query holod->interface list

[rwestphal]

For 1, it means that holo-cli should be able to query holod->interface list

Correct. I think the best way to do that would be by adding a new gRPC RPC like SystemInfo. That RPC would return the list of available interfaces at the OS-level, and maybe additional data as well, such as the available tunneling technologies.

I'll transfer this issue to the holo-cli repo since it's primarily CLI work.