Right now we can't create subscription on a topic in a different gcp
project since it assume the project from the subscription. The provider
always create the full topic name string
projects/{project}/topics/{topic} with the received topic property.
Using a regexp we validate if the string is already in
the format projects/{project}/topics/{topic} and if it's the case
we don't wrap it again and take it directly. The original functionality
is maintained but it's possible to specify a different project for the
topic.