GSocketConnectable describing the identity of the server that
is expected on the other end of the connection.
G_TLS_CERTIFICATE_BAD_IDENTITY flag is set in
GTlsClientConnection:validation-flags, this object will be used
to determine the expected identify of the remote end of the
GTlsClientConnection:server-identity is not set,
or does not match the identity presented by the server, then the
G_TLS_CERTIFICATE_BAD_IDENTITY validation will fail.
In addition to its use in verifying the server certificate, this is also used to give a hint to the server about what certificate we expect, which is useful for servers that serve virtual hosts.