auth: Support SHA2 extension for pubkey authentication (RFC 8332)

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 82da0c3361)

include/libssh/pki.h

@@ -96,6 +96,10 @@ typedef struct ssh_signature_struct *ssh_signature;
 ssh_key ssh_key_dup(const ssh_key key);
 void ssh_key_clean (ssh_key key);
 
+const char *
+ssh_key_get_signature_algorithm(ssh_session session,
+                                enum ssh_keytypes_e type);
+
 /* SSH Signature Functions */
 ssh_signature ssh_signature_new(void);
 void ssh_signature_free(ssh_signature sign);