Add support for instance sharing

[datevfink]

There are services that allow instance sharing. This way an instance can be used by two spaces at the same time. This is especially useful for Messaging Queues.

I found no configuration support in the current cloudfoundry_service_instance resource and would be glad if this changed in future

see: https://docs.cloudfoundry.org/devguide/services/sharing-instances.html

[LitschiW]

see also https://github.com/cloudfoundry-community/terraform-provider-cloudfoundry/issues/481