This commit includes
- Removing CoAP sources from mbed-client-c to its own repository.
- mbed-client-c is moved outside of mbed-os source tree because it is used by mbed-client, so
it will be part of mbed-client offering.
- CoAP sources are used by coap-service which is a part of nanostack deliveries hence coap now existing
as independent module under mbed-coap.
- Commit handles the interdepdency of coap-service with mbed-client-c because of coap sources, coap-service
header files are now fixed to point to right header directory.