Support reusing existing models 

## Feature Request
Users can specify a directory path containing existing models so to reuse pre-generated models. For example:
```
kcl-openapi generate model -f <spec_file> -t <target_dir> --existing-models foo/bar
```

some scenarios of how users might use this:
- Users might want to add some custom KCL models converted from custom crds while reusing the existing k8s builtin models
