[tls] Generate signatures using an explicitly supported algorithm

Generate the CertificateVerify signature using a signature hash
algorithm from the list provided by the server, rather than inferring
an algorithm that is likely to be supported based on the certificate's
public-key algorithm and the key schedule's digest algorithm.

This adds support for the within-handshake variant of client
authentication as used by TLS version 1.3, which requires the use of
RSA-PSS for certificates with an RSA public key.

It is also likely to change the algorithm selected for TLS version 1.2
client authentication: if the server advertises (and prefers) the
RSA-PSS signature algorithms then we will now use RSA-PSS rather than
RSA-PKCS#1.

Signed-off-by: Michael Brown <mcb30@ipxe.org>