net/handshake: Add a kernel API for requesting a TLSv1.3 handshake

To enable kernel consumers of TLS to request a TLS handshake, add
support to net/handshake/ to request a handshake upcall.

This patch also acts as a template for adding handshake upcall
support for other kernel transport layer security providers.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/handshake.yaml

@@ -16,7 +16,7 @@ definitions:
     type: enum
     name: handler-class
     value-start: 0
-    entries: [ none, max ]
+    entries: [ none, tlshd, max ]
   -
     type: enum
     name: msg-type
@@ -120,3 +120,5 @@ mcast-groups:
   list:
     -
       name: none
+    -
+      name: tlshd