Here we add a new resource type `gitlab_project_hook`. It allows for
management of custom hooks for a gitlab project.
This is a relatively simple resource as a project hook is a simple
association between a project, and a url to hit when one of the flagged
events occurs on that project.
Hooks (called Webhooks in some user documentation, but simply Hooks
in the api documentation) are covered here for users
https://docs.gitlab.com/ce/user/project/integrations/webhooks.html and
in the API documentation at
https://docs.gitlab.com/ce/api/projects.html#hooks
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