Add macro and function for choosing message layer

[yuhaoth]

Suggested enhancement

MPS(message process stack) has been created and apply in TLS1.3 code. It will replace legay message layer in future.
But TLS1.2 still use legacy message layer now.

we need MACRO and function to identify which case should use legacy layer and which should use mps.

define a new macro MBEDTLS_SSL_LEGACY_MSG_LAYER_REQUIRED

#if defined(MBEDTLS_SSL_PROTO_TLS1_2) || \
    ( defined(MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL) && !defined(MBEDTLS_SSL_USE_MPS) )
#define MBEDTLS_SSL_LEGACY_MSG_LAYER_REQUIRED
#endif

For runtime dispatch, define a static inline function like mbedtls_ssl_legacy_msg_layer_in_use()