feat(plugin-form-builder)!: update form builder plugin field overrides to use a function instead (#6497)

## Description

Changes the `fields` override for form builder plugin to use a function
instead so that we can actually override existing fields which currently
will not work.

```ts
//before
fields: [
  {
    name: 'custom',
    type: 'text',
  }
]

// current
fields: ({ defaultFields }) => {
  return [
    ...defaultFields,
    {
      name: 'custom',
      type: 'text',
    },
  ]
}
```

## Type of change

- [x] Breaking change (fix or feature that would cause existing
functionality to not work as expected)

test/plugin-form-builder/config.ts

@@ -73,12 +73,26 @@ export default buildConfigWithDefaults({
         //   singular: 'Contact Form',
         //   plural: 'Contact Forms'
         // },
-        fields: [
-          {
-            name: 'custom',
-            type: 'text',
-          },
-        ],
+        fields: ({ defaultFields }) => {
+          return [
+            ...defaultFields,
+            {
+              name: 'custom',
+              type: 'text',
+            },
+          ]
+        },
+      },
+      formSubmissionOverrides: {
+        fields: ({ defaultFields }) => {
+          return [
+            ...defaultFields,
+            {
+              name: 'custom',
+              type: 'text',
+            },
+          ]
+        },
       },
       redirectRelationships: ['pages'],
     }),