Merge development into tls13-prototype, adjusting example programs

[hanno-becker]

In upstream Mbed TLS, there has been a major code restructuring regarding the example programs: A lot of shared code between ssl_client2 and ssl_server2 has been moved into a new source file.

Before making further changes to the example programs, we should merge development into tls13-prototype in order to make sure we're not diverging further from the new way the example programs are handled in upstream Mbed TLS.