// ParseAllowedSigners parses an entry in the format of the allowed_signers file.
//
// The allowed_signers format is documented in the ssh-keygen(1) manual page.
// This function will parse a single entry from in. On successful return,
// principals will contain the list of principals that this entry matches,
// options will contain the list of options that this entry matches (i.e.
// "cert-authority", "namespaces=file,git"), and pubKey will contain the
// public key. See the ssh-keygen(1) manual page for the various forms that a
// principal string can take, and further details on the options.
//
// The unparsed remainder of the input will be returned in rest. This function
// can be called repeatedly to parse multiple entries.
//
// If no entries were found in the input then err will be io.EOF. Otherwise, a
// non-nil err value indicates a parse error.
func ParseAllowedSigners(in []byte) (principals []string, options []string, pubKey ssh.PublicKey, rest []byte, err error)