keymgr: Support storing x25519 keys in OpenSSH format

ArtiNativeKeyStore needs to be able to store x25519 keys in OpenSSH format. However, x25519 OpenSSH keys are not currently supported by the ssh-key crate (which is what we're parsing OpenSSH keys with).

As a workaround, in #900 (closed) we decided that in the Arti key store, x25519 private keys will actually be ed25519 keys, which are converted to x25519 upon loading (using the conversion function added in !1297 (merged)). This approach isn't good enough though: most people will probably want to bring their existing x25519 keys, and in order to store those in OpenSSH format, we'd need convert them to ed25519, which is impossible (because the secret part of an x25519 key contains a SHA512'd secret, whereas the corresponding, "un-expanded", ed25519 secret key contains the secret itself rather than the SHA).