When the join field is used, Payload now automatically adds an index on
the target relationship field.
For example:
```
{
name: 'relatedPosts',
type: 'join',
collection: "posts",
on: 'category',
},
{
name: 'category',
type: 'relationship',
relationTo: "categories",
},
```
Here, `index: true` implicitly added to the `category` relationship
field during sanitization to improve querying performance.
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