[12.x] Add operator class support for PostgreSQL GiST spatial indexes

[joteejotee]

This PR adds support for specifying operator classes when creating spatial indexes on PostgreSQL, addressing issue #56261.

Problem

PostgreSQL GiST indexes require explicit operator classes for certain data types (like inet). Currently, Laravel's spatialIndex() method doesn't support specifying operator classes, causing migrations to fail with:

ERROR: data type inet has no default operator class for access method "gist"

Solution

Add an optional $operatorClass parameter to Blueprint::spatialIndex():

// Before (fails for inet)
$table->spatialIndex(['coordinates']);

// After (works with operator class)
$table->spatialIndex(['coordinates'], 'my_index', 'point_ops');
$table->spatialIndex(['client_internet'], 'inet_idx', 'inet_ops');

Changes

• Add optional $operatorClass parameter to Blueprint::spatialIndex()
• Update PostgresGrammar::compileSpatialIndex() to handle operator classes
• Add comprehensive tests for the new functionality
• Maintain full backward compatibility

Fixes #56261