Array of authorities that are accepted for client certificates.
The server will only accept connections from clients whose certificate
is signed by one of these authorities. The string is a DER-encoded certificate,
which typically must be decoded using MIME.decode_base64 or
Tools.PEM.Msg first.
Note that it is presumed that the issuer will also be trusted by the server. See
trusted_issuers for details on specifying trusted issuers.
If empty, the server will accept any client certificate whose issuer is trusted by the
server.