Ability to configure User-Agent for URL previews

[alexgleason]

Hello. I would like the ability to configure the user-agent used in link preview requests, with an option like: url_preview_user_agent

Why: because currently links to YouTube do not render a link preview on Synapse. This is YouTube's fault. However you can bypass it by pretending to be Facebook, by setting the user-agent to facebookexternalua. Then YouTube link previews will work properly again.

Exposing a configuration will let server admins work around the issue.

[MadLittleMods]

Closing per this reasoning from the associated PR,

Discussed with the team and we're not keen on pretending to be someone else. If someone doesn't want to serve content to Synapse, that's their prerogative.

This might be a XY problem. The goal is make YouTube URL previews work. Do they have some API/flow that they prefer people to use? Should we be using OpenGraph/oEmbed instead of scraping, etc? We'd rather have PR's for that instead of this workaround.

-- @MadLittleMods, #17968 (comment)

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@clokep also shared the canonical issue we're using to track YouTube URL previews not working -> #17462