tor-keymgr: Refactor code shared between ArtiNativeKeystore and ArtiEphemeralKeystore

This MR contains some keymgr refactorings, mostly in the form of code motion/reorganisation.

The point of this refactoring is to tackle a small bit of tech debt: the ephemeral keystore impl no longer needs to use types and functions meant for the on-disk ArtiNativeKeystore, because we now have an SshKeyData.into_ephemeral() function that can be used by either keystore to convert the shared storage format (i.e. the SshKeyData returned by EncodableKey::as_ssh_key_data) to an erased key type that can be used by the KeyMgr.

Closes #1362 (closed) #1367 (closed)