Summary

This adds the ability to reflect PostgreSQL index access methods (gin, gist, spgist, brin, hash) when describing indexes.

Changes:

• Add accessMethod property to Index class with constants for PostgreSQL access methods (GIN, GIST, SPGIST, BRIN, HASH)
• Update describeIndexQuery() to join pg_am and fetch amname
• Update describeIndexes() and convertIndexDescription() to include accessMethod in result for non-btree indexes
• Update indexSql() to generate USING clause for non-btree indexes

Why:

This enables migrations (cakephp/migrations#1053) to properly reflect and regenerate indexes that use specialized PostgreSQL access methods. Without reflection support, snapshot generation would be lossy - indexes created with GIN/GIST/etc would be reflected as standard btree indexes.

Usage

When an index uses a non-btree access method, the reflected index data will include the accessMethod field:

// Example reflected GIN index
[
    'name' => 'articles_tags_idx',
    'type' => 'index',
    'columns' => ['tags'],
    'accessMethod' => 'gin',
]

The Index class now includes constants for PostgreSQL access methods:

Index::GIN    // 'gin'
Index::GIST   // 'gist'
Index::SPGIST // 'spgist'
Index::BRIN   // 'brin'
Index::HASH   // 'hash'

Refs cakephp/migrations#1053