Dynamically fetch oembed provider metadata (for url previews)

[ShadowJonathan]

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 #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.

[anoadragon453]

For clarity, Synapse already supports oEmbed previews as of #7920, but this is currently limited to a list of non-configurable providers (e.g twitter).

This issue suggests having the configuration be fetched from https://oembed.com/providers.json, which is updated regularly and recommended by the oEmbed spec.

Sounds like a pretty good idea to me, honestly.

[clokep]

I think there was a discussion somewhere (maybe in #7920?) of allowing users to add entries to the list via the config file too. A bit orthogonal to this issue, but thought it was worth mentioning.

[richvdh]

related: #9733, #2752