spacelift_space_by_path should support relative paths

Currently the Spacelift Terraform resource spacelift_space_by_path requires an absolute path (from root on down). This restriction makes it difficult to create position-independent code. It would preferable to support “space_path” query constraints in the form of relative paths. Paths could be interpreted as relative if they don’t start with “root”. By default relative paths should be resolved with respect to the current space (the one returned by spacelift_current_space). An additional enhancement would be to add a new query constraint called “parent_space_id” that defined the space against which the relative path is resolved.