Add Migrations.default_collation config option

Allow configuring a global default collation for MySQL tables via
Configure::read('Migrations.default_collation'). When null (default),
tables inherit the database server collation. Per-table collation
options still take precedence.

Author: dereuromark

config/app.example.php

@@ -9,5 +9,6 @@
         'unsigned_primary_keys' => null, // Default false
         'unsigned_ints' => null, // Default false, make sure this is aligned with the above config
         'column_null_default' => null, // Default false
+        'default_collation' => null, // Default null (uses database collation). Set to e.g. 'utf8mb4_unicode_ci' to override.
     ],
 ];

src/Db/Adapter/MysqlAdapter.php

@@ -253,6 +253,11 @@ public function createTable(TableMetadata $table, array $columns = [], array $in
             'engine' => 'InnoDB',
         ];
 
+        $collation = Configure::read('Migrations.default_collation');
+        if ($collation) {
+            $defaultOptions['collation'] = $collation;
+        }
+
         $options = array_merge(
             $defaultOptions,
             array_intersect_key($this->getOptions(), $defaultOptions),