Implement RawPublicKey security mode for CoAP

Description:

Currently security based on RawPublicKeys (see https://tools.ietf.org/html/rfc7252#section-9) is not implemented for CoAP, because the underlying network libraries have not implemented it yet. In particular, according to https://en.wikipedia.org/wiki/Comparison_of_TLS_implementations it seems like OpenSSL does not plan to add the implementation anytime soon. If we want to support it, there are 2 options:

• Wait until RawPublicKey mode is implemented by the underlying libraries of Qt Network. As soon as Qt Network supports it, QCoapQUdpConnection can implement the RawPublicKey mode.
• Investigate other DTLS implementations supporting this mode already and integrate in Qt Coap module (by adding an alternative implementation of the QCoapConnection interface). A possible option could be using a transport based on TinyDTLS (https://projects.eclipse.org/projects/iot.tinydtls), which is widely used by other secure CoAP implementations.