[memory] use heap for messages (#5792)

This commit adds an option to allow share heap memory with message
buffer pool.

For MTD, there is usually no border agent or commissioner service. This
change allows the heap for meshcop to be used by application or network
management data after the device is attached. Thus the total memory
usage can be reduced.

script/check-size

@@ -120,6 +120,7 @@ size_nrf52840_version()
         "JOINER=1"
         "LINK_RAW=1"
         "MAC_FILTER=1"
+        "MESSAGE_USE_HEAP=1"
         "MTD_NETDIAG=1"
         "SERVICE=1"
         "SLAAC=1"