Add API for TLS server PSK callback

[GregBurns]

mbedtls has an API mbedtls_ssl_conf_psk_cb() for requesting the PSK for a specific identifier. This functionality is not currently exposed in Zephyr. The mbedtls_ssl_conf_psk() API exposed via setsockopt only allows a single identifier to be associated with the PSK.

This is the application note from the mbedtls documentation for mbedtls_ssl_conf_psk() :

Note
This is mainly useful for clients. Servers will usually want to use mbedtls_ssl_conf_psk_cb() instead.
Currently clients can only register one pre-shared key. In other words, the servers' identity hint is ignored.

An interim solution would be to provide an API to obtain a pointer to the mbedtls_ssl_config struct for a socket so the application could make the appropriate call to mbedtls. This would at least address porting of existing applications that currently make use of mbedtls_ssl_conf_psk_cb()