Make the constructor automatically set the discriminator value

[eucyt]

Resolve #733

Changes

Currently, when creating a polymorphic model, you have to set the discriminator value manually each time, even though the value is obvious.

$message = (new TextMessage())
  ->setType(\LINE\Constants\MessageType::TEXT) // this is redundant.
  ->setText('hello!');

With this implementation, the discriminator value will be automatically set in the constructor.

$message = (new TextMessage())
  ->setText('hello!');

$this->assertEquals('text', $message->getType()); // Passed

For reviewers

Since this commit contains many auto-generated files, I recommend reviewing it.

Add tests
Fix template

[habara-k]

Thank you!