feat: Future-proof `NodeAddr`

[flub]

The current NodeAddr looks like this:

pub struct NodeAddr {
    pub node_id: NodeId,
    pub relay_url: Option<RelayUrl>,
    pub direct_addresses: BTreeSet<SocketAddr>,
}

However internally we have refactored things into Transports and conceptually the home relay is not that a special a transport. In fact you could easily allow for an iroh endpoint to be reachable via multiple relay servers, perhaps a public one and one reachable only on some private network.

We should make sure that the NodeAddr is more agnostic towards transports.