Add support for PostgreSQL index access methods (gist, spgist, brin, hash)

[dereuromark]

Summary

This PR adds support for additional PostgreSQL index access methods beyond the existing GIN support:

• GiST (Generalized Search Tree) - for geometric data, range types, full-text search
• SP-GiST (Space-Partitioned GiST) - for data with natural clustering (IP addresses, phone numbers)
• BRIN (Block Range Index) - for large, naturally-ordered tables like time-series data
• Hash - for simple equality comparisons

Additionally, this adds support for specifying operator classes (opclass) on index columns, which is useful for specialized index configurations like trigram similarity searches with gist_trgm_ops.

Changes

• Add Index::GIN, Index::GIST, Index::SPGIST, Index::BRIN, Index::HASH constants
• Add opclass option to Index class for specifying operator classes
• Generalize PostgresAdapter::getIndexSqlDefinition() to support all access methods
• Add comprehensive tests for each new index type
• Update documentation with examples for all PostgreSQL index access methods

Usage

// GIN index for JSONB
$table->addIndex('tags', ['type' => 'gin']);

// GiST index with operator class for trigram search
$table->addIndex('name', [
    'type' => 'gist',
    'opclass' => ['name' => 'gist_trgm_ops'],
]);

// BRIN index for time-series data
$table->addIndex('recorded_at', ['type' => 'brin']);

// SP-GiST index for IP addresses
$table->addIndex('client_ip', ['type' => 'spgist']);

// Hash index for equality lookups
$table->addIndex('session_id', ['type' => 'hash']);

Refs cakephp/phinx#1823