[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('[email protected]'));
$this->assertTrue($mailable->hasCc('[email protected]'));
$this->assertTrue($mailable->hasBcc('[email protected]'));
$this->assertTrue($mailable->hasReplyTo('[email protected]'));
$this->assertTrue($mailable->hasReplyTo('[email protected]'));
$this->assertTrue($mailable->hasFrom('[email protected]'));
$this->assertTrue($mailable->hasTag('test'));
$this->assertTrue($mailable->hasMetadata('foo', 'bar'));

After:

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

$mailable
	->assertHasTo('[email protected]')
	->assertHasCc('[email protected]')
	->assertHasBcc('[email protected]')
	->assertHasReplyTo('[email protected]')
	->assertHasReplyTo('[email protected]')
	->assertHasFrom('[email protected]')
	->assertHasTag('test')
	->assertHasMetadata('foo', 'bar');