Data source gitlab_project_branches should support regex and search properties

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Data source gitlab_project_branches has only one input parameter, project. But related Gitlab REST API suports optional paramters regex and search. Would be great if this data source will have the same functionality.

Implementation Guide

• Read the CONTRIBUTING.md guide to see how to set up your local development environment.
• Add regex and search to the datasource schema as optional attributes.
• Add regex and search to the ListBranchesOptions struct if they are non-nil and non-unknown.
• Add datasource tests using the two new options.