Previously, the Arti key store would store x25519 secret keys as ed25519
OpenSSH keys, which it would convert 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).
Now that `ssh-key` has support for ssh keys with [custom algorithm
names], we can store x25519 in OpenSSH format directly. This commit
changes the storage format used by the keymgr for x25519 client auth
keys (from ed25519-ssh to our own custom key type with an algorithm name
of `"x25519@torproject.org"`).
Closes#936
[custom algorithm names]: https://github.com/RustCrypto/SSH/pull/136
17d965e894