Relaxed semantics for PSK configuration

This affects users which call the PSK configuration APIs mbedtlsl_ssl_conf_psk() and mbedtls_ssl_conf_psk_opaque() multiple times on the same SSL configuration.

In Mbed TLS 2.x, users would observe later calls overwriting the effect of earlier calls, with the prevailing PSK being the one that has been configured last. In Mbed TLS 3.0, calling mbedtls_ssl_conf_[opaque_]psk() multiple times will return an error, leaving the first PSK intact.

To achieve equivalent functionality when migrating to Mbed TLS 3.0, users calling mbedtls_ssl_conf_[opaque_]psk() multiple times should remove all but the last call, so that only one call to either mbedtls_ssl_conf_psk() or mbedtls_ssl_conf_psk_opaque() remains.