Support for device specific keys

[octol]

This PR adds support for a new data store that is intended to be used in the vpn client. More precisely:

• Create new crate nym-vpn-store that supports storing device specific keys.
• Create the keys as part of connecting with nym-vpnd.

The device keys are similar but orthogonal to the existing keys used by the mixnet client.

Things not covered that we'll need to add later:

• command to vpnd to query the keypair