net: openthread: added heap related otPlat methods implementation

* Added implementations of otPlatCAlloc and otPlatFree methods
necessary for the OpenThread in case of using EXTERNAL_HEAP.
* Added CONFIG_OPENTHREAD_DNSSD_SERVER option to allow enabling
OT_DNSS_SERVER feature.

Signed-off-by: Kamil Kasperczyk <kamil.kasperczyk@nordicsemi.no>