binary/Hexadecimal PSK support

[jvermillard]

The Client expects the PSK value to be a string, so I tried to set my hex string there, but debugging it, I saw the string is converted to bytes before being sent to the DTLS implementation.

I would expect the PSK value to be entered as Hex and not a string, as mandated by rfc4279: https://datatracker.ietf.org/doc/html/rfc4279#section-5.4
Also, how the current implementation would cope with non-ASCII/UTF8 generated binary PSKs?