Mysql dialect support for createFunction and dropFunction

What are you doing?

// Using a Mysql server...
(sequelize) => {
  const queryInterface = sequelize.getQueryInterface();

  queryInterface.dropFunction('TEST_FUNC').finally(() => {
    queryInterface.createFunction(
      'TEST_FUNC',
      [{name: 'test_param', type: 'VARCHAR(100)', direction: 'IN'}],
      'VARCHAR(100)',
      'SQL',
      'RETURN test_param;',
      ['DETERMINISTIC']
    ).then(
      () => console.log('TEST_FUNC created'),
      (error) => { throw error }
    );
  });
}

What do you expect to happen?

Mysql database acquires the function TEST_FUNC.

What is actually happening?

TypeError: this.QueryGenerator.createFunction is not a function

…and, indeed, there is no createFunction in sequelize/lib/dialects/mysql/query-generator.js.

I needed this for my project, please find my patch for 5.0.0-beta.10 attached.

Dialect: mysql
Dialect version: (5.0.0-beta.10)
Database version: any
Sequelize version: 5.0.0-beta.10
Tested with latest release: Yes

Start in sequelize/lib/dialects/mysql/query-generator.js and compare the existing dialect implementations of createFunction and dropFunction. Verify the reported sample against MySQL; done means the MySQL query interface supports both operations without the reported TypeError and creates and drops TEST_FUNC.