feat(database): support unsigned primary keys on MySQL

[sjorsvanleeuwen]

Summary

Adds an unsigned option to CreateTableStatement::primary() so integer primary keys can be created as INTEGER UNSIGNED on MySQL:

new CreateTableStatement('books')
    ->primary(unsigned: true)
    ->foreignId('author_id', constrainedOn: 'authors', unsigned: true)
    ->text('title');

MySQL rejects a foreign key whose column signedness differs from the referenced primary key, so belongsTo() and foreignId() also get an unsigned parameter that is passed through to the underlying integer column.

PostgreSQL and SQLite have no unsigned integer type. In line with #2295, the primary key output for those dialects is unchanged.

Changes

• PrimaryKeyStatement: new unsigned flag; emits INTEGER UNSIGNED PRIMARY KEY AUTO_INCREMENT on MySQL.
• CreateTableStatement: unsigned parameter on primary(), belongsTo() and foreignId().
• Tests for unsigned primary keys and unsigned foreign keys across all dialects.
• Docs: new "Using unsigned integers as primary keys" section.

All parameters default to false, so existing migrations are unaffected.

[sjorsvanleeuwen]

Feel free to move this to a discussion on Discord :)