Reapply "blueprintload: enable strict checking for toml"

This reverts commit f25b5e325e.

internal/blueprintload/blueprintload.go

@@ -21,11 +21,13 @@ func decodeToml(r io.Reader, what string) (*externalBlueprint.Blueprint, error)
 	dec := toml.NewDecoder(r)
 
 	var conf externalBlueprint.Blueprint
-	_, err := dec.Decode(&conf)
-
+	metadata, err := dec.Decode(&conf)
 	if err != nil {
 		return nil, fmt.Errorf("cannot decode %q: %w", what, err)
 	}
+	if len(metadata.Undecoded()) > 0 {
+		return nil, fmt.Errorf("cannot decode %q: unknown keys found: %v", what, metadata.Undecoded())
+	}
 
 	return &conf, nil
 }