feat: persist p2p identities

[dignifiedquire]

The identity of the p2p node should be persistet between restarts.
As we want to be able to use multiple identites on the same node, this should be given some thought, as not to assume a singular identity.

Storage should likely be abstract over a trait, with an in memory version (which would replicate the current behaviour and will be useful for testing), an on disk version (just flat files like .ssh) and a iroh-store version, in where the identities are stored in a specific part of the database instance.

[dignifiedquire]

fixed in n0-computer/iroh#125