net: tls: use Maximum Fragment Length (MFL) extension by default

Call mbedtls_ssl_conf_max_frag_len() on created TLS context
configuration, so that Maximum Fragment Length (MFL) will be sent to
peer using RFC 6066 max_fragment_length extension. MFL value is
automatically chosen based on MBEDTLS_SSL_OUT_CONTENT_LEN and
MBEDTLS_SSL_IN_CONTENT_LEN mbed TLS macros.

This extension is mostly useful for TLS client side to tell TLS server
what is the maximum supported receive record length.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>