Support for DTLS certificate export about go-coap

[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.