Support parsing OpenSSH authorized_keys files

PR #29 adds a foundation for parsing single keys, which is useful for parsing public key files. It would be helpful for NIOSSH servers to implement an authorized_keys parser.

Start by reading PR #29, which provides the single-key parsing foundation, and inspect how that work is exposed for public key files. Define the authorized_keys parsing entry point and expected supported records, then verify that NIOSSH servers can consume the parsed keys.

Written by the indexing model from the issue text.