Add definitions of the new DCM ext API

Add definitions of the new DCM ext API

New API is supposed to give DCM backends possibility
to define any custom method guarded with a Cynara privilege.

New API is not mandatory for plugins - plugins can implement
old, new, or both APIs of DCM.

This commit makes header files available for both: client
and the backend.

The DCM EXT error codes are defined in separate header
for easier management of errors returned from the daemon
to client.

Change-Id: Ia3478d1352f6eabfa913a975f7ee09c02b976939