Issue description

Openstack Orchestration API allows for a "dry-run" creation of a stack with the /preview endpoint : https://docs.openstack.org/api-ref/orchestration/v1/#preview-stack

Openstack4j could provide support fro this endpoint by adding a preview() method to the StackService java API.
Classes that would need to be extended :
https://github.com/openstack4j/openstack4j/blob/master/core/src/main/java/org/openstack4j/api/heat/StackService.java
https://github.com/openstack4j/openstack4j/blob/master/core/src/main/java/org/openstack4j/openstack/heat/internal/StackServiceImpl.java

Submitter checklist

Make sure that following is specified to make the issue easier to process:

• The issue description links to OpenStack API documentation of that particular endpoint/request (https://docs.openstack.org/api-quick-start/#current-api-versions).
• OpenStack service name is used as an issue name prefix (ex.: "Neutron: Add floatingIp port forwardings service").
• If applicable, add GitHub link to classes that needs extending.