Provider implementation of connectivity attachment

Addition of a implementation of new allotted resource "connectivity
attachment" to generic-resource-api schema. Connectivity attachment
will be provided by Connectivity service and will be allotted to Site
service as part of CCVPN use case. This new allotted-resource can serve
for both SOTNAttachment and SDWANAttachment.

Change-Id: I1823249b6ab9fe315e83862abcf05be65ceded5a
Issue-ID: SDNC-384
Signed-off-by: Gaurav Agrawal <gaurav.agrawal@huawei.com>