Add TOSCA policy status header field to APEX event create CLI

Add TOSCA policy status header field to APEX event create CLI

In order to expose the processing status of a TOSCA policy
to APEX-PDP, a new header field for APEX concept "event"
has been introduced to track this within an APEX policy
implementation. This field will be leveraged to extract
the TOSCA policy execution metrics. Note, that the field is
marked as optional for backward compatibility.
Unit tests have been augmented to test for the field wherever
applicable and the default empty value is retained for the others.

"example-grpc" module has been updated to include this field.
Exposing the TOSCA policy execution metrics is outside the
scope of this patch since the current changes are already
very large owing to the atomic nature of the change introduced.

Issue-ID: POLICY-3845
Signed-off-by: Rashmi Pujar <rashmi.pujar1@bell.ca>
Change-Id: Ief6d70f9abcfc8414e10aa51a27815ee9028e4c8