Support for in-band image link previews

https://xmpp.org/extensions/inbox/link-metadata.html allows:

    <og:image>https://link.to.example.com/image.png</og:image>
    <og:image>cid:sha-256+e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855@bob.xmpp.org</og:image>
    <og:image>ni:///shai-256;47DEQpj8HBSa-_TImW-5JCeuQeRkm5NMpJWZG3hSuFU</og:image>
    <og:image>data:image/jpeg,...</og:image>
    <og:video>https://videos.example.com/video-embed.html</og:video>

We probably don't want to fetch an external image (we could make that opt-in though). But in-band bob or base64-encoded jpeg would be nice to have. Recent Cheogram does send that already, it seems (to be verified).