ssh-key: fix parsing comments with whitespace in AuthorizedKeys

[mkeeter]

ssh_key::PublicKey::read_openssh_file can read the following key:

ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBN76zuqnjypL54/w4763l7q1Sn3IBYHptJ5wcYfEWkzeNTvpexr05Z18m2yPT2SWRd1JJ8Aj5TYidG9MdSS5J78= hello world

However, if I treat it as a single-line authorized_keys and read it with ssh_key::AuthorizedKeys::read_file, it fails:

  --- stderr
  Error: failed to read authorized keys from: "authorized_keys"

  Caused by:
      0: Base64 encoding error: invalid Base64 encoding
      1: Base64 encoding error: invalid Base64 encoding
      2: invalid Base64 encoding

This is because the authorized_keys parser doesn't respect spaces in the comment field, so it's trying to parse ecdsa-sha2-nistp256 as the options and hello as the key material.

---

This PR fixes the issue, and also adds a unit test for authorized_keys with whitespace in their comments.