Choose proper namespaces for macros and options introduced in prototype

[hanno-becker]

This is issue is about going through all the macros and compile-time options introduced by the prototype and making sure they live in appropriate namespaces. For most options, it will likely be MBEDTLS_SSL_, though we could also consider MBEDTLS_SSL_TLS1_3_ if suitable.

Example: MBEDTLS_ZERO_RTT