support branch merge targets workflow

New Resource

add support for creating workflow merge target rules as documented here https://docs.gitlab.com/ee/user/project/repository/branches/#configure-workflows-for-target-branches

resource "gitlab_project_merge_workflow_rule" "feature" {
    project        = "namespace/myproject"
    source_pattern = "feature/*"
    target_branch  = "develop"
}

-->

Related GitLab APIs

I'm unable to locate the API documentation

Additional Details

• GitLab REST API resources available (read for data sources, crud for resources)
• go-gitlab does support the related GitLab API already
• I'd like to contribute it myself