download: add a hook to rewrite URLs

[StefanKarpinski]

It would often be handy to be able to intercept URLs that are downloaded with Base.download to be able to redirect them elsewhere. Having an official hook mechanism rather than having to monkey patch Base.download would make this easier and less fragile.

[oxinabox]

I feel like advanced functionality, is better served by HTTP.download.
Though idk that that case is handled there yet.
Maybe worth opening an issue for it?

[StefanKarpinski]

See #32804. Pkg uses BinaryProvider.download, but we want to merge that into Base.download and in a controlled environment, it's important to be able to redirect attempts to download things from the internet to internal replacement locations. If we move HTTP into stdlib then we could rely on HTTP.download for this, but I don't see why this couldn't be part of Base.download.

[vtjnash]

triage seems largely OK with proceeding with this as not a big risk and we were convinced it'll be very useful to a small subset of users