feat: route to invidious companion on downloads

[alexmaras]

This sets the download URL to invidious companion (as enabled in iv-org/invidious-companion#41) when companion is used.

There is a secondary way of approaching this - proxying the download through invidious. However, it looks like the CompanionConnectionPool is set up to only have a short timeout, and a download endpoint will need to allow for long requests.

Note that this approach embeds the companion URL in the page, which means the check param would eventually expire. It looks like that's 6 hours though, so I'm not sure that's too much of a concern.

There's also a couple of extra bits needed in companion to make this work well, all in iv-org/invidious-companion#95
That PR does 2 things:

1. chunks up downloads so that companion isn't throttled by youtube servers
2. ensures the check query param is passed on to the latestVersion call that's made so that it doesn't error out.

Without that PR merged, this still works, but only if verify_request is false in invidious-companion.

Addresses #5218