Archive URL preview content at the time it is posted to a room

This issue has been migrated from [#10676](https://github.com/matrix-org/synapse/issues/10676).

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When a user posts a URL in Slack, the preview at the time is saved in the channel. This can be very handy for sites with dynamic content, so someone going back to the channel history can see at least a snippet of the content available from the URL at the time of the posting. It's likewise useful for URLs that disappear over the time. If the user posted a Twitter link into the channel, the entire Tweet appears there, which allows for archival access when tweets are deleted. This stored preview information is also preserved when a Slack Workspace is exported, but there is no way to import that information into Matrix.

It would be very helpful for Matrix to be able to generate and store the same information for URL previews as a configurable option. This would also enable a user migrating a Slack Workspace to import this metadata and not lose it when transitioning from Slack to Matrix.