New Entity ORM Method groupByCallback

[techmahedy]

This PR introduces a new groupByCallback() method to the Entity ORM query builder.
The method enables grouping records using custom, application-level logic, rather than being limited to database columns.

This is especially useful when grouping depends on computed values, ranges, or domain-specific rules that cannot be expressed directly in SQL.

Example: Group Users by Age Range

$groups = User::query()
  ->groupByCallback(function ($user) {
      if ($user->age < 18) return 'minor';
      if ($user->age < 65) return 'adult';
      return 'senior';
  });

Result structure:

[
    'minor'  => Collection,
    'adult'  => Collection,
    'senior' => Collection,
]