Add TestEmail tag/metadata access/assertions

[kbond]

For use with: https://symfony.com/doc/current/mailer.html#adding-tags-and-metadata-to-emails

$testEmail->tags(); // array of metadata tag values
$testEmail->tag(); // first tag or null if none
$testEmail->metadata(); array of metadata
$testEmail
    ->assertHasTag('password-reset')
    ->assertHasMetadata('user-id') // assert that metadata key exists
    ->assertHasMetadata('color', 'blue') // assert that metadata key exists and is expected value
;