Added gitlab_branch as a resource and data source

Created by: eddb7

I have moved this pr here as my branch in the previous PR was accidentally progressed.

Original message: Hi I have been using this provider and required a way to automate and manage the creation and deletion of gitlab branches. This allows the management of a gitlab_branch by adding it as a resource or data source block in terraform The create, delete and read methods are using: https://github.com/xanzy/go-gitlab/blob/master/branches.go#L188

The data structure for the resources is modelled on the response from https://docs.gitlab.com/ee/api/branches.html#get-single-repository-branch

The properties around branch protection have been excluded for consistency, allowing this to work alongside the gitlab_branch_protection resources

It would be great to get some feedback and thoughts about wether this could potentially be integrated into the main package