Trigger to create a preview environment

This ticket that used to be "Restrictions rules for preview environments triggering" has now been changed to another solution, "Trigger to create a preview environment" as we decided that if you want to be able to specify directly in the PR when a PR should create a preview environment instead of building complex rules within Qovery. So the objective of this task is now to add a flow within your PR that will allow you to trigger the creation of the preview environment whenever you want instead of having it created by default at each PR