PreviewUrlResource: set `od:iframe` in json (#2752)

[srdjan-catalyst]

Set `od:iframe` in json when oembed returns `<iframe/>`

Signed-off-by: Srdjan <[email protected]>

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[clokep]

@srdjan-catalyst Can you expand upon why this is necessary / what this does?

[srdjan-catalyst]

Currently, when oembed api returns html, it is parsed and as far as content is concerned, only images are being looked for. We have a case when an iframe is sent back to display content. That is because of the content being available to authenticated users only so it has to be dealt with on the client side.

[erikjohnston]

This appears blocked on https://github.com/matrix-org/matrix-doc/pull/3338 now?