Add fixed option for binary column type

[dereuromark]

Summary

• Adds support for the fixed attribute on binary columns to distinguish between fixed-length BINARY and variable-length VARBINARY types
• When reflecting a BINARY(n) column from the database, fixed => true is now passed through to the Column object
• When baking migrations, the fixed option is now included in the generated migration code

This is the companion PR to cakephp/cakephp#19207 which adds the core framework support for this feature.

Usage

// Creates VARBINARY(20) - variable length (default)
$table->addColumn('data', 'binary', ['length' => 20]);

// Creates BINARY(20) - fixed length  
$table->addColumn('hash', 'binary', ['length' => 20, 'fixed' => true]);

Refs #999

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Constraint: This one doesnt technically need one.

Also, even if, since #1013 is raising the constraint already, I skipped it for this PR here.