perf(ui): only select the useAsTitle field when fetching the document data of a relationship value (#13228)

### What?
Optimize the relationship value loading by selecting only the
`useAsTitle` field when fetching document data via the REST API.

### Why?
Previously, all fields were fetched via a POST request when loading the
document data of a relationship value, causing unnecessary data transfer
and slower performance. Only the `useAsTitle` field is needed to display
the related document’s title in the relationship UI field.

### How?
Applied a select to the REST API POST request, similar to how the
options list is loaded, limiting the response to the `useAsTitle` field
only.

Author: jhb-dev

packages/ui/src/fields/Relationship/Input.tsx

@@ -413,11 +413,17 @@ export const RelationshipInput: React.FC<RelationshipInputProps> = (props) => {
       })
 
       if (idsToLoad.length > 0) {
+        const collection = getEntityConfig({ collectionSlug: relation })
+        const fieldToSelect = collection?.admin?.useAsTitle || 'id'
+
         const query = {
           depth: 0,
           draft: true,
           limit: idsToLoad.length,
           locale,
+          select: {
+            [fieldToSelect]: true,
+          },
           where: {
             id: {
               in: idsToLoad,
@@ -436,8 +442,6 @@ export const RelationshipInput: React.FC<RelationshipInputProps> = (props) => {
             },
             method: 'POST',
           })
-
-          const collection = getEntityConfig({ collectionSlug: relation })
           let docs = []
 
           if (response.ok) {