[Scaffolder] Get data of other fields in Form from a custom field

Allows to use e.g.
```
const CustomFieldExtensionComponent(props: FieldExtensionComponentProps<string[]>) => {
     const { formContext } = props;
     ....
};

const CustomFieldExtension = scaffolderPlugin.provide(
  createScaffolderFieldExtension({
    name: ...,
    component: CustomFieldExtensionComponent,
    validation: ....
  })
);
```

Signed-off-by: Minh Trí <tritri251214@gmail.com>

docs/features/software-templates/writing-custom-field-extensions.md

@@ -161,3 +161,20 @@ spec:
           description: My custom name for the component
           ui:field: MyCustomExtension
 ```
+
+## Get data of other fields in Form from a Custom Field Extension
+
+```tsx
+const CustomFieldExtensionComponent(props: FieldExtensionComponentProps<string[]>) => {
+  const { formContext } = props;
+  ...
+};
+
+const CustomFieldExtension = scaffolderPlugin.provide(
+  createScaffolderFieldExtension({
+    name: ...,
+    component: CustomFieldExtensionComponent,
+    validation: ...
+  })
+);
+```