Support GitLab group resource with full path

### What problem are you facing?
I want to create a observe-only GitLab group resource, but I only know the full path of the group, but not the ID.

### How could Crossplane help solve your problem?
Similar to the GitLab API and the terraform provider for GitLab, support lookup via the full path and not only the ID.

Example:
```
apiVersion: groups.gitlab.crossplane.io/v1alpha1
kind: Group
metadata:
name: team-a-observed
annotations:
crossplane.io/external-name: platform/team-a # At the moment only a numeric ID is supported here
spec:
managementPolicies:
- Observe
```