Externalizes more subscription parameters (#783)

* Externalizes more subscription parameters

Allows :
- to parametrize the polling interval of subscription
- to enable the setting to queue the subscription jobs immediately.

Following the change in hapifhir/hapi-fhir/pull/6395

* Fix formatting

src/main/resources/application.yaml

@@ -309,6 +309,8 @@ hapi:
 #    subscription:
 #      resthook_enabled: true
 #      websocket_enabled: false
+#      polling_interval_ms: 5000
+#      immediately_queued: false
 #      email:
 #        from: some@test.com
 #        host: google.com