feat: join field across many collections (#10919)

This feature allows you to specify `collection` for the join field as
array.
This can be useful for example to describe relationship linking like
this:
```ts
{
  slug: 'folders',
  fields: [
    {
      type: 'join',
      on: 'folder',
      collection: ['files', 'documents', 'folders'],
      name: 'children',
    },
    {
      type: 'relationship',
      relationTo: 'folders',
      name: 'folder',
    },
  ],
},
{
  slug: 'files',
  upload: true,
  fields: [
    {
      type: 'relationship',
      relationTo: 'folders',
      name: 'folder',
    },
  ],
},
{
  slug: 'documents',
  fields: [
    {
      type: 'relationship',
      relationTo: 'folders',
      name: 'folder',
    },
  ],
},
```

Documents and files can be placed to folders and folders themselves can
be nested to other folders (root folders just have `folder` as `null`).

Output type of `Folder`:
```ts
export interface Folder {
  id: string;
  children?: {
    docs?:
      | (
          | {
              relationTo?: 'files';
              value: string | File;
            }
          | {
              relationTo?: 'documents';
              value: string | Document;
            }
          | {
              relationTo?: 'folders';
              value: string | Folder;
            }
        )[]
      | null;
    hasNextPage?: boolean | null;
  } | null;
  folder?: (string | null) | Folder;
  updatedAt: string;
  createdAt: string;
}
```

While you could instead have many join fields (for example
`childrenFolders`, `childrenFiles`) etc - this doesn't allow you to
sort/filter and paginate things across many collections, which isn't
trivial. With SQL we use `UNION ALL` query to achieve that.

---------

Co-authored-by: Dan Ribbens <dan.ribbens@gmail.com>

test/joins/config.ts

@@ -220,6 +220,120 @@ export default buildConfigWithDefaults({
         },
       ],
     },
+    {
+      slug: 'multiple-collections-parents',
+      fields: [
+        {
+          type: 'join',
+          name: 'children',
+          collection: ['multiple-collections-1', 'multiple-collections-2'],
+          on: 'parent',
+          admin: {
+            defaultColumns: ['title', 'name', 'description'],
+          },
+        },
+      ],
+    },
+    {
+      slug: 'multiple-collections-1',
+      admin: { useAsTitle: 'title' },
+      fields: [
+        {
+          type: 'relationship',
+          relationTo: 'multiple-collections-parents',
+          name: 'parent',
+        },
+        {
+          name: 'title',
+          type: 'text',
+        },
+        {
+          name: 'name',
+          type: 'text',
+        },
+      ],
+    },
+    {
+      slug: 'multiple-collections-2',
+      admin: { useAsTitle: 'title' },
+      fields: [
+        {
+          type: 'relationship',
+          relationTo: 'multiple-collections-parents',
+          name: 'parent',
+        },
+        {
+          name: 'title',
+          type: 'text',
+        },
+        {
+          name: 'description',
+          type: 'text',
+        },
+      ],
+    },
+
+    {
+      slug: 'folders',
+      fields: [
+        {
+          type: 'relationship',
+          relationTo: 'folders',
+          name: 'folder',
+        },
+        {
+          name: 'title',
+          type: 'text',
+        },
+        {
+          type: 'join',
+          name: 'children',
+          collection: ['folders', 'example-pages', 'example-posts'],
+          on: 'folder',
+          admin: {
+            defaultColumns: ['title', 'name', 'description'],
+          },
+        },
+      ],
+    },
+    {
+      slug: 'example-pages',
+      admin: { useAsTitle: 'title' },
+      fields: [
+        {
+          type: 'relationship',
+          relationTo: 'folders',
+          name: 'folder',
+        },
+        {
+          name: 'title',
+          type: 'text',
+        },
+        {
+          name: 'name',
+          type: 'text',
+        },
+      ],
+    },
+    {
+      slug: 'example-posts',
+      admin: { useAsTitle: 'title' },
+      fields: [
+        {
+          type: 'relationship',
+          relationTo: 'folders',
+          name: 'folder',
+        },
+        {
+          name: 'title',
+          type: 'text',
+        },
+        {
+          name: 'description',
+          type: 'text',
+        },
+      ],
+    },
   ],
   localization: {
     locales: [