[coap] add new helper to allocate and init CoAP message (#7631)

This commit adds new helper methods in `Coap` to allocate a new
message and initialize it as a confirmable or non-confirmable post to
a given URI path. It also adds a new helper method to allocate a CoAP
response message for a given request. These methods help simplify the
preparation of CoAP and TMF messages.

src/core/thread/tmf.cpp

@@ -85,7 +85,7 @@ Error Agent::Start(void)
     return Coap::Start(kUdpPort, OT_NETIF_THREAD);
 }
 
-Error Agent::Filter(const ot::Coap::Message &aMessage, const Ip6::MessageInfo &aMessageInfo, void *aContext)
+Error Agent::Filter(const Message &aMessage, const Ip6::MessageInfo &aMessageInfo, void *aContext)
 {
     OT_UNUSED_VARIABLE(aMessage);