BUG/MEDIUM: ssl: chosing correct certificate using RSA-PSS with TLSv1.3

BUG/MEDIUM: ssl: chosing correct certificate using RSA-PSS with TLSv1.3

The clienthello callback was written when TLSv1.3 was not yet out, and
signatures algorithm changed since then.

With TLSv1.2, the least significant byte was used to determine the
SignatureAlgorithm, which could be rsa(1), dsa(2), ecdsa(3).
https://datatracker.ietf.org/doc/html/rfc5246#section-7.4.1.4.1

This was used to chose which type of certificate to push to the client.

But TLSv1.3 changed that, and introduced new RSA-PSS algorithms that
does not have the least sinificant byte to 1.
https://datatracker.ietf.org/doc/html/rfc8446#section-4.2.3

This would result in chosing the wrong certificate when an RSA an ECDSA
ones are in the configuration for the same SNI or default entry.

This patch fixes the issue by parsing bothe hash and signature field to
check the RSA-PSS signature scheme.

This must fix issue #2852.

This must be backported in every stable versions. The code was moved
from ssl_sock.c to ssl_clienthello in recent versions.

(cherry picked from commit 3912780b1ee656b6d7327c34027f7b5d9db61731)
Signed-off-by: William Lallemand <wlallemand@haproxy.com>