MINOR: ssl: Add new 'pha' server option

MINOR: ssl: Add new 'pha' server option

This backend option allows post handshake authentication for the given
backend. This simply consists in adding a "post_handshake_auth"
extension in the ClientHello that tells the server that PHA is
supported. We only enable it for backends that have an explicit
certificate provided so that this certificate gets always used in
subsequent CertificateRequest messages sent by the server.
This does not work with AWS-LC (PHA unsupported) or WolfSSL which would
support it but only when WolfSSL is built with
WOLFSSL_POST_HANDSHAKE_AUTH defined (disabled by default).