feat: add widgets schema validation (#3841)

packages/netlify-cms-widget-relation/src/index.js

@@ -1,10 +1,12 @@
 import controlComponent from './RelationControl';
 import previewComponent from './RelationPreview';
+import schema from './schema';
 
 const Widget = (opts = {}) => ({
   name: 'relation',
   controlComponent,
   previewComponent,
+  schema,
   ...opts,
 });
 

packages/netlify-cms-widget-relation/src/schema.js

@@ -0,0 +1,12 @@
+export default {
+  properties: {
+    collection: { type: 'string' },
+    valueField: { type: 'string' },
+    searchFields: { type: 'array', minItems: 1, items: { type: 'string' } },
+    file: { type: 'string' },
+    multiple: { type: 'boolean' },
+    displayFields: { type: 'array', minItems: 1, items: { type: 'string' } },
+    optionsLength: { type: 'integer' },
+  },
+  required: ['collection', 'valueField', 'searchFields'],
+};