feat(plugin-search)!: make search collection fields override into a function that provides defaultFields inline with other plugins (#7095)

searchPlugin's searchOverrides for the collection now takes in a fields
function instead of array similar to other plugins and patterns we use
to allow you to map over existing fields as well if needed.

```ts
// before
searchPlugin({
  searchOverrides: {
    slug: 'search-results',
    fields: [
      {
        name: 'excerpt',
        type: 'textarea',
        admin: {
          position: 'sidebar',
        },
      },
    ]
  },
}),

// current
searchPlugin({
  searchOverrides: {
    slug: 'search-results',
    fields: ({ defaultFields }) => [
      ...defaultFields,
      {
        name: 'excerpt',
        type: 'textarea',
        admin: {
          position: 'sidebar',
        },
      },
    ]
  },
}),
```

test/plugin-search/config.ts

@@ -41,14 +41,14 @@ export default buildConfigWithDefaults({
         posts: ({ title }) => (title === 'Hello, world!' ? 30 : 20),
       },
       searchOverrides: {
-        fields: [
+        fields: ({ defaultFields }) => [
+          ...defaultFields,
           {
             name: 'excerpt',
-            type: 'text',
+            type: 'textarea',
             admin: {
-              readOnly: true,
+              position: 'sidebar',
             },
-            label: 'Excerpt',
           },
         ],
       },