Migrate dagger.#Secret and dagger.#Stream to new format

- Refactored to keep every transformation of built-in types (e.g. FS,
  Secret, etc) to/from CUE in the same place (plancontext)
- dagger.#Service and dagger.#Secret are now following the new FS-like format
  (e.g. `_service: id: string`)
- Backward compatibility
- dagger.#Stream is now an alias for dagger.#Service

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

docs/reference/kubernetes/README.md

@@ -37,7 +37,7 @@ Apply Kubernetes resources
 |*url*             | `*null \| string`         |Kubernetes manifest url to deploy remote configuration    |
 |*namespace*       | `*"default" \| string`    |Kubernetes Namespace to deploy to                         |
 |*version*         | `*"v1.19.9" \| string`    |Version of kubectl client                                 |
-|*kubeconfig*      | `dagger.#Secret`          |Kube config file                                          |
+|*kubeconfig*      | `(string\|struct)`        |Kube config file                                          |
 
 ### kubernetes.#Resources Outputs