Dynamically fetch `oembed` provider metadata (for url previews)

This issue has been migrated from [#9877](https://github.com/matrix-org/synapse/issues/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 https://github.com/matrix-org/synapse/issues/9733#issuecomment-814111058_

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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.