Dynamically fetch oembed provider metadata (for url previews)

[matrixbot]

This issue has been migrated from #9877.

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it might be nice to be able to specify custom hooks in synapse's configuration, although I'm not sure if that's really worth the effort.

This shouldn't be too hard, it would also be nice to default to using the documented providers (https://oembed.com/providers.json).

Originally posted by @clokep in matrix-org/synapse#9733 (comment)

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Fetching this, and refreshing every week or so, could give better experience to users for embeds, I think adding this would help a lot with miscellaneous embedding problems users might not report, and instead "magically" make each one of them work for their favourite websites.