authorized_keys.py: accept ssh-ed25519 and similar future types
authorized_keys.py: accept ssh-ed25519 and similar future types

Unfortunately, both options and key algos/types have practically the
same basic syntax (\w+) so the only two ways to distinguish between them
are:

- whitelist known types (or at least type prefixes) as the code is
  currently doing, or

- search for the first token that starts with the string ("\0\0\0" +
  prev_token).encode("base64"), then go backwards by 1.

The second option is more reliable but also somewhat ugly.

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