feat: join field with polymorphic relationships (#9990)

### What?
The join field had a limitation imposed that prevents it from targeting
polymorphic relationship fields. With this change we can support any
relationship fields.

### Why?
Improves the functionality of join field.

### How?
Extended the database adapters and removed the config sanitization that
would throw an error when polymorphic relationships were used.

Fixes #

test/joins/collections/Categories.ts

@@ -115,6 +115,30 @@ export const Categories: CollectionConfig = {
       collection: 'posts',
       on: 'blocks.category',
     },
+    {
+      name: 'polymorphic',
+      type: 'join',
+      collection: postsSlug,
+      on: 'polymorphic',
+    },
+    {
+      name: 'polymorphics',
+      type: 'join',
+      collection: postsSlug,
+      on: 'polymorphics',
+    },
+    {
+      name: 'localizedPolymorphic',
+      type: 'join',
+      collection: postsSlug,
+      on: 'localizedPolymorphic',
+    },
+    {
+      name: 'localizedPolymorphics',
+      type: 'join',
+      collection: postsSlug,
+      on: 'localizedPolymorphics',
+    },
     {
       name: 'singulars',
       type: 'join',