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

This issue has been migrated from [#10547](https://github.com/matrix-org/synapse/pull/10547).

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Set `od:iframe` in json when oembed returns ``

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>

### Pull Request Checklist

<!-- Please read CONTRIBUTING.md before submitting your pull request -->

* [X] Pull request is based on the develop branch
* [X] Pull request includes a [changelog file](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.md#changelog). The entry should:
- Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
- Use markdown where necessary, mostly for `code blocks`.
- End with either a period (.) or an exclamation mark (!).
- Start with a capital letter.
* [X] Pull request includes a [sign off](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.md#sign-off)
* [X] Code style is correct (run the [linters](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.md#code-style))