Streamline ease of use with relation subquery (#33180)

Since the Relation forwards calls to the query builder object, it can be turned into a subquery just like a regular query builder object

src/Illuminate/Database/Query/Builder.php

@@ -8,6 +8,7 @@
 use Illuminate\Database\Concerns\BuildsQueries;
 use Illuminate\Database\ConnectionInterface;
 use Illuminate\Database\Eloquent\Builder as EloquentBuilder;
+use Illuminate\Database\Eloquent\Relations\Relation;
 use Illuminate\Database\Query\Grammars\Grammar;
 use Illuminate\Database\Query\Processors\Processor;
 use Illuminate\Pagination\Paginator;
@@ -337,7 +338,7 @@ protected function createSub($query)
      */
     protected function parseSub($query)
     {
-        if ($query instanceof self || $query instanceof EloquentBuilder) {
+        if ($query instanceof self || $query instanceof EloquentBuilder || $query instanceof Relation) {
             return [$query->toSql(), $query->getBindings()];
         } elseif (is_string($query)) {
             return [$query, []];
@@ -3086,6 +3087,7 @@ protected function isQueryable($value)
     {
         return $value instanceof self ||
                $value instanceof EloquentBuilder ||
+               $value instanceof Relation ||
                $value instanceof Closure;
     }