fix(drizzle): indexes / unique with relationships (#8432)

Fixes https://github.com/payloadcms/payload/issues/8413 and
https://github.com/payloadcms/payload/issues/6460

- Builds indexes for relationships by default in the SQL schema
- Fixes `unique: true` handling with Postgres / SQLite for every type of
relationships (non-polymorphic. hasMany, polymorphic, polymorphic
hasMany) _note_: disables unique for nested to arrays / blocks
relationships in the `_rels` table.
- adds tests

2.0 PR tha ports only indexes creation
https://github.com/payloadcms/payload/pull/8446, because `unique: true`
could be a breaking change if someone has incosistent unique data in the
database.

test/fields/payload-types.ts

@@ -1020,6 +1020,29 @@ export interface IndexedField {
   id: string;
   text: string;
   uniqueText?: string | null;
+  uniqueRelationship?: (string | null) | TextField;
+  uniqueHasManyRelationship?: (string | TextField)[] | null;
+  uniqueHasManyRelationship_2?: (string | TextField)[] | null;
+  uniquePolymorphicRelationship?: {
+    relationTo: 'text-fields';
+    value: string | TextField;
+  } | null;
+  uniquePolymorphicRelationship_2?: {
+    relationTo: 'text-fields';
+    value: string | TextField;
+  } | null;
+  uniqueHasManyPolymorphicRelationship?:
+    | {
+        relationTo: 'text-fields';
+        value: string | TextField;
+      }[]
+    | null;
+  uniqueHasManyPolymorphicRelationship_2?:
+    | {
+        relationTo: 'text-fields';
+        value: string | TextField;
+      }[]
+    | null;
   uniqueRequiredText: string;
   localizedUniqueRequiredText: string;
   /**