feat(resource_group): add endpoint to get the current job of a resource group

What

Adds ResourceGroupService.GetCurrentJobForAResourceGroup for GET /projects/:id/resource_groups/:key/current_job.

It returns the *Job currently holding the resource group. testing/resource_group_mock.go is regenerated.

Tests

TestResourceGroups_GetCurrentJobForAResourceGroup covers success, an invalid ID type, a request option error and a 404.

Related

Closes #2317 (closed)

GitLab API docs: https://docs.gitlab.com/api/resource_groups/#retrieve-current-job-for-a-resource-group