Add ARP to Datadog Service Catalog

[nihil2501]

Use accredited-representatives to match this statement in our application controller:

# Add this `service_tag` to the Datadog Service Catalog.
#   https://app.zenhub.com/workspaces/accredited-representative-facing-team-65453a97a9cc36069a2ad1d6/issues/gh/department-of-veterans-affairs/va.gov-team/77004
# It will be the dd-service property for your application here:
#   https://github.com/department-of-veterans-affairs/vets-api/tree/master/datadog-service-catalog
service_tag 'accredited-representatives'

Need to review datadog requirements and determine if any additional work needs to be completed on the ARF side.