ssl: issue TLS session tickets for server-side sockets

A server-side SSLSocket did a full handshake for every connection. Browsers
open several connections per page, and on a small MCU with an RSA-2048
certificate each handshake costs ~1.3 s of CPU (ESP32-C6), so pages stalled
and speculative connections were abandoned.

Configure mbedTLS session tickets (AES-256-GCM via PSA, 24 h lifetime, one
process-wide key) on server-side contexts when MBEDTLS_SSL_SESSION_TICKETS and
MBEDTLS_SSL_TICKET_C are available (ESP-IDF enables them by default), so
returning clients resume with an abbreviated handshake and no public-key
operation. Client-side behaviour is unchanged.

Author: tyeth

shared-module/ssl/SSLSocket.c

@@ -21,6 +21,7 @@
 #include "shared-bindings/socketpool/enum.h"
 
 #include "mbedtls/version.h"
+#include "mbedtls/ssl_ticket.h"
 
 #define MP_STREAM_POLL_RDWR (MP_STREAM_POLL_RD | MP_STREAM_POLL_WR)
 
@@ -327,6 +328,23 @@ ssl_sslsocket_obj_t *common_hal_ssl_sslcontext_wrap_socket(ssl_sslcontext_obj_t
             goto cleanup;
         }
     }
+    #if defined(MBEDTLS_SSL_SESSION_TICKETS) && defined(MBEDTLS_SSL_TICKET_C)
+    if (server_side) {
+        // Session tickets let a returning client resume without the (expensive) public-key
+        // handshake. One process-wide ticket key, rotated by lifetime; nothing to persist.
+        static mbedtls_ssl_ticket_context ticket_ctx;
+        static bool ticket_ready = false;
+        if (!ticket_ready) {
+            mbedtls_ssl_ticket_init(&ticket_ctx);
+            if (mbedtls_ssl_ticket_setup(&ticket_ctx, PSA_ALG_GCM, PSA_KEY_TYPE_AES, 256, 86400) == 0) {
+                ticket_ready = true;
+            }
+        }
+        if (ticket_ready) {
+            mbedtls_ssl_conf_session_tickets_cb(&o->conf, mbedtls_ssl_ticket_write, mbedtls_ssl_ticket_parse, &ticket_ctx);
+        }
+    }
+    #endif
     return o;
 cleanup:
     mbedtls_pk_free(&o->pkey);