Bug 38390: Add subscriptions+count to vendors embed

Bug 38390: Add subscriptions+count to vendors embed

This patch adds the subscriptions+count option to the embeddable enum
for the /vendors endpoint.

Test plan
1) Inspect the change to t/db_dependant/api/v1/acquisitions_vendors.t
   and confirm it's testing for the addition of subscriptions_count.
2) Run the above tests and confirm it passes

Note: For the above tests to pass, you will need to ensure your api
bundle is rebuilt after the patches are applied and restart plack.
`yarn build && restart_all` in ktd

Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>