Support for DTLS certificate export

[vidavidorra]

I couldn't find this to be supported ATM, but how difficult would it be to support this or is this already supported?

I'm also still figuring out in pion/dtls how i can export certificates, so once that is figured out, this ticket should become more specific. Nonetheless, I figured creating a ticket here to see if anyone has done anything similar, and get the ball rolling once I've figured out how to export the certificate from pion/dtls.
After a fist look (and I'm not sure whether this is indeed right until the dtls is figured out) it might mean that the dtls.Conn needs to be exposed somehow from coap.ListenAndServeDTLS.

[jdbruijn]

I've got everything in the DTLS figured out, now need to find out how to do this in coap.
In pion/dtls I can now do the following to get the remote certificate, where conn is of type net.Conn.

conn, err := listener.Accept()
util.Check(err)
// defer conn.Close() // TODO: graceful shutdown

dtlsConn := conn.(*dtls.Conn)
cert := dtlsConn.RemoteCertificate()
fmt.Println("Remote certificate: \n", cert)

I'm currently trying to get to know the go-coap codebase a little bit better. @jkralik do you have any pointers to get me started?
In the end I'd need to get the (DTLS) remote certificate in the handler functions (e.g. handleA in the DTLS server example.

I'm thinking to add a a field for net.Conn to HandlerFunc or to Request. I've tried to understand it a bit and initially thought I'd be able to use req.Client.srv but req.Client only exposes its functions. When I logged req.Client I saw that srv is nil and only commander is set to a value (&{<nil> <nil> 0xc000201350 <nil> false}).
I'll keep looking further but would love some guidance on how to do this and in general whether you'd want something like this in the code. (I'ld need it to pass the client certificates through to another process so I'd need to fork this or something if you don't want this in the code base).

BTW: I'm the same user as @vidavidorra , but am currently in the process of changing that to an organisation so from now on will be using this user.