Add support for unfurling a wider range of websites. Most code changes are
related to the implementation of a new Unfurler, an OEmbedUnfurler, which is
necessary to get metadata for Reddit URLs using oEmbed, since Reddit does not
support OpenGraph meta tags. The new unfurler will also be useful for other
websites, like Twitter. Also the user agent was changed, and now more websites
consider status-go reasonably human.
Related to issue https://github.com/status-im/status-mobile/issues/15918
Example hostnames that are now unfurleable: reddit.com, open.spotify.com,
music.youtube.com
Other improvements:
- Better error handling, especially because I wasn't wrapping errors correctly.
I also removed the unnecessary custom error UnfurlErr.
- I made tests truly deterministic by parameterizing the http.Client instance
and by customizing its Transport field (except for some failing conditions
where it's even good to hit the real servers).
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