Add data sources gitlab_project_protected_branch(es)

[sirlatrom]

Fixes #549.

[armsnyder]

Can you fix these test failures please?

=== RUN   TestAccDataGitlabProjectProtectedBranchSearch
Error:     testing.go:684: Step 0 error: errors during plan:
        
        Error: "branch": required field is not set
        
          on /tmp/tf-test957964864/main.tf line 8:
          (source code not available)
        
        
--- FAIL: TestAccDataGitlabProjectProtectedBranchSearch (0.22s)
=== RUN   TestAccDataGitlabProjectProtectedBranchesSearch
Error:     testing.go:684: Step 0 error: errors during plan:
        
        Error: "branch": required field is not set
        
          on /tmp/tf-test150129522/main.tf line 8:
          (source code not available)
        
        
--- FAIL: TestAccDataGitlabProjectProtectedBranchesSearch (0.20s)

By the way there are new instructions in the README for running acceptance tests you could try out.

[sirlatrom]

@armsnyder Acceptance tests fixed, thanks for pointing them out.

[timofurrer]

@nagyv This merge caused master to fail, see the fixes here: #785

[github-actions]

This functionality has been released in 3.9.0 of the Terraform GitLab Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue. Thank you!