Add new config MBEDTLS_SSL_CONTEXT_SERIALIZATION

This is enabled by default as we generally enable things by default unless
there's a reason not to (experimental, deprecated, security risk).

We need a compile-time option because, even though the functions themselves
can be easily garbage-collected by the linker, implementing them will require
saving 64 bytes of Client/ServerHello.random values after the handshake, that
would otherwise not be needed, and people who don't need this feature
shouldn't have to pay the price of increased RAM usage.

programs/ssl/query_config.c

@@ -1218,6 +1218,14 @@ int query_config( const char *config )
     }
 #endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
 
+#if defined(MBEDTLS_SSL_CONTEXT_SERIALIZATION)
+    if( strcmp( "MBEDTLS_SSL_CONTEXT_SERIALIZATION", config ) == 0 )
+    {
+        MACRO_EXPANSION_TO_STR( MBEDTLS_SSL_CONTEXT_SERIALIZATION );
+        return( 0 );
+    }
+#endif /* MBEDTLS_SSL_CONTEXT_SERIALIZATION */
+
 #if defined(MBEDTLS_SSL_DEBUG_ALL)
     if( strcmp( "MBEDTLS_SSL_DEBUG_ALL", config ) == 0 )
     {