feat: add opengraph properties to preview_url spec

[cperrin88]

The Matrix spec doesn't specify enough opengraph properties to make the preview_url endpoint useful. This patch adds some usefule properties.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[lukaslihotzki-f]

This seems to be generally useful, so please open a PR upstream first: https://github.com/matrix-org/matrix-spec

[cperrin88]

They went with a different route in the spec. They just linked to the OpenGraph Spec: matrix-org/matrix-spec#1753

Would you consider this PR again now?