MBEDTLS_SSL_TLS1_3_COMPATIBILITY_MODE macro

\def MBEDTLS_SSL_TLS1_3_COMPATIBILITY_MODE Enable TLS 1.3 middlebox compatibility mode. As specified in Section D.4 of RFC 8446, TLS 1.3 offers a compatibility mode to make a TLS 1.3 connection more likely to pass through middle boxes expecting TLS 1.2 traffic. Turning on the compatibility mode comes at the cost of a few added bytes on the wire, but it doesn't affect compatibility with TLS 1.3 implementations that don't use it. Therefore, unless transmission bandwidth is critical and you know that middlebox compatibility issues won't occur, it is therefore recommended to set this option. Comment to disable compatibility mode for TLS 1.3. If MBEDTLS_SSL_PROTO_TLS1_3 is not enabled, this option does not have any effect on the build.