feat(plugin-redirects)!: update fields overrides to use a function (#6675)

## Description

Updates the `fields` override in plugin redirects to allow for
overriding

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

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


## Type of change

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

test/plugin-redirects/config.ts

@@ -27,6 +27,17 @@ export default buildConfigWithDefaults({
   plugins: [
     redirectsPlugin({
       collections: ['pages'],
+      overrides: {
+        fields: ({ defaultFields }) => {
+          return [
+            ...defaultFields,
+            {
+              type: 'text',
+              name: 'customField',
+            },
+          ]
+        },
+      },
     }),
   ],
 })