TLS 1.3 client and server: Early data support preparatory work

[ronald-cron-arm]

• Add and document the MBEDTLS_SSL_EARLY_DATA configuration option (preferred to MBEDTLS_ZERO_RTT in the prototype, disputable) in mbedtls_config.h.
• Define and document 'early_data_enabledfield inmbedtls_ssl_config`
• Define and document mbedtls_ssl_conf_early_data() to enable/disable the early data on client and server side: just the simplest signature mbedtls_ssl_conf_early_data( mbedtls_ssl_config *conf, int early_data_enabled ) for the time being. Depending on the result of Define the API for writing early data on client side in TLS 1.3 #4902 and Define the API to read handshake data on server side in TLS 1.3 #6310 this may evolve but not in the scope of this issue.