Handle structured data (JSON-LD / Microdata / RDFa) for URL previews

This issue has been migrated from [#11540](https://github.com/matrix-org/synapse/issues/11540).

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[Google supports "structured data"](https://developers.google.com/search/docs/advanced/structured-data/intro-structured-data) which is possibly fairly prevalent as it is used for SEO:

> Google uses structured data that it finds on the web to understand the content of the page, as well as to gather information about the web and the world in general.

The format they support are:

* [JSON-LD](https://json-ld.org/)
* [Microdata](https://html.spec.whatwg.org/multipage/)
* [RDFa](https://rdfa.info/)

Support for this would probably be similar to what we do for oEmbed and would essentially boil down to an {JSON-LD|Microdata|RDFa}->Open Graph converter.

See a conversation around this: https://matrix.to/#/!NasysSDfxKxZBzJJoE:matrix.org/$tvfeceE24AkQJyxB8Oj82Qm8g1dPpyKB8aE3pMS-nH4?via=matrix.org&via=libera.chat&via=envs.net