fix: add generic type support for CmsWidget and CmsWidgetParam (#6018)

Author: iChenLei

packages/netlify-cms-core/index.d.ts

@@ -462,16 +462,16 @@ declare module 'netlify-cms-core' {
     fieldsMetaData: Map<string, any>;
   }
 
-  export interface CmsWidgetParam {
+  export interface CmsWidgetParam<T = any> {
     name: string;
-    controlComponent: CmsWidgetControlProps;
-    previewComponent?: CmsWidgetPreviewProps;
+    controlComponent: CmsWidgetControlProps<T>;
+    previewComponent?: CmsWidgetPreviewProps<T>;
     globalStyles?: any;
   }
 
-  export interface CmsWidget {
-    control: CmsWidgetControlProps;
-    preview?: CmsWidgetPreviewProps;
+  export interface CmsWidget<T = any> {
+    control: CmsWidgetControlProps<T>;
+    preview?: CmsWidgetPreviewProps<T>;
     globalStyles?: any;
   }