[9.x] Add mailable assertions

[macbookandrew]

Currently, the Mailable class has a number of methods to determine whether the mailable has the given recipient, cc, bcc, attachments, etc.

The mailable includes a couple of direct assertions for attachments ($mail->assertHasAttachment(), etc.), but not for recipients, tags, or metadata.

This PR introduces the missing assertions:

Before:

$mailable = (new Mail())->build();

$this->assertTrue($mailable->hasTo('taylor@otwell.com'));
$this->assertTrue($mailable->hasCc('taylor@otwell.com'));
$this->assertTrue($mailable->hasBcc('taylor@otwell.com'));
$this->assertTrue($mailable->hasReplyTo('taylor@otwell.com'));
$this->assertTrue($mailable->hasReplyTo('taylor@otwell.com'));
$this->assertTrue($mailable->hasFrom('taylor@otwell.com'));
$this->assertTrue($mailable->hasTag('test'));
$this->assertTrue($mailable->hasMetadata('foo', 'bar'));

After:

$mailable = (new Mail())->build();

$mailable
	->assertHasTo('taylor@otwell.com')
	->assertHasCc('taylor@otwell.com')
	->assertHasBcc('taylor@otwell.com')
	->assertHasReplyTo('taylor@otwell.com')
	->assertHasReplyTo('taylor@otwell.com')
	->assertHasFrom('taylor@otwell.com')
	->assertHasTag('test')
	->assertHasMetadata('foo', 'bar');