DTLS 1.3 config flag

[theodorsm]

This issue is part of a larger effort to implement an MVP of DTLS 1.3 (see parrent issue #727). We should have a config flag for enabling DTLS 1.3. The DTLS 1.3 implementation should keep the handshake/flight logic separate from the existing DTLS 1.2 implementation.

[JoTurk]

We should make use of the Pion config / settings patterns, best example is the settings engine https://github.com/pion/webrtc/blob/master/settingengine.go we're slowly changing every API to this pattern, example rtp and ice