Use configMapRef for ConfigMap data sources

Update the `MCPRegistry` data types to use existing kubernetes data types like `configMapRef`.

Change from:
```yaml
spec:
  source:
    type: configmap
    configmap:
      name: minimal-registry-data
```

to:
```yaml
spec:
  source:
    type: configmap
    configMapRef:
      name: minimal-registry-data
```
