Support for Craft letterbox image transform mode

[john-henry]

The “Letterbox” (letterbox) image transform mode was added in Craft 4.4.
https://github.com/craftcms/cms/blob/develop/CHANGELOG.md#development-1

It would be great if Image Optimize could support this.

[birdkiwi]

It might be useful for creating fixed-size og:images, so that they can 'contain' an image inside. Please add letterbox mode 🙏