ot::Coap::CoapBase::NewConfirmablePostMessage() method

Allocates and initializes a new CoAP Confirmable Post message with normal priority level. The CoAP header is initialized as `kTypeConfirmable` and `kCodePost` with a given URI and a randomly generated token (of default length). This method also sets the payload marker (calling `SetPayloadMarker()`). Even if message has no payload, calling `SetPayloadMarker()` is harmless, since `SendMessage()` will check and remove the payload marker when there is no payload.