Add GET request handler for CRED resource

Add GET request handler for CRED resource

GET request returns credential without private data
This patch is requried, because there is step for 'GET oic/sec/cred' in TEST SPEC.

Patch 1: Init patch.
Patch 2: Add OC/SRPGetCredResource API for debugging only.
Patch 3: Apply Randeep's comment

Change-Id: I3ff154f786a278b6737e86d66b47624abd8e2434
Signed-off-by: Dmitriy Zhuravlev <d.zhuravlev@samsung.com>
Signed-off-by: js126.lee  <js126.lee@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/8397
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Randeep Singh <randeep.s@samsung.com>