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>

client/client.go

@@ -217,7 +217,7 @@ func (c *Client) logSolveStatus(ctx context.Context, pctx *plancontext.Context,
 	secureSprintf := func(format string, a ...interface{}) string {
 		s := fmt.Sprintf(format, a...)
 		for _, secret := range secrets {
-			s = strings.ReplaceAll(s, secret.PlainText, "***")
+			s = strings.ReplaceAll(s, secret.PlainText(), "***")
 		}
 		return s
 	}