[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>

.changeset/scaffolder-form-context.md

@@ -0,0 +1,21 @@
+---
+'@backstage/plugin-scaffolder': minor
+---
+
+Get data of other fields in Form from a custom field in template Scaffolder.
+following:
+
+```tsx
+const CustomFieldExtensionComponent(props: FieldExtensionComponentProps<string[]>) => {
+  const { formContext } = props;
+  ...
+};
+
+const CustomFieldExtension = scaffolderPlugin.provide(
+  createScaffolderFieldExtension({
+    name: ...,
+    component: CustomFieldExtensionComponent,
+    validation: ...
+  })
+);
+```

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: ...
+  })
+);
+```

plugins/scaffolder/src/components/MultistepJsonForm/MultistepJsonForm.tsx

@@ -198,6 +198,7 @@ export const MultistepJsonForm = (props: Props) => {
                   widgets={widgets}
                   noHtml5Validate
                   formData={formData}
+                  formContext={formData}
                   onChange={onChange}
                   onSubmit={e => {
                     if (e.errors.length === 0) handleNext();