Support brotli-encoded feeds

[Seirdy]

NextCloud News currently supports fetching feeds with on-the-fly gzip compression. It should support decompressing brotli-encoded feeds too.

On the NextCloud server's end: unpaginated full-text feeds are often quite large, and users often have hundreds of feeds. On the feed provider's end, all NextCloud News instances may end up fetching a single Atom feed thousands of times per day. Small savings add up fast.

I'm not a PHP dev. I do see that NextCloud News uses curl to fetch feeds. libcurl supports Brotli compression in almost every distro, so adding support should be fairly straightforward.

[Grotax]

I think that can be done, we basically use guzzle+feed-io for the feeds

For guzzle it is simply a question of adding the header (which we already have for gzip) with the right value so in this case add 'br' I think.

I'm not so familiar with how widespread the support for brotli is.

To my understanding PHP must be new enough (we require 8.0 now) and libcurl must support it.

If we can expect all major distros to support it, I think we can simply add 'br' to the list.

I'm not sure what would happen if we add br and then the system does not support it maybe the connection would fail.

But if we can say that is available almost everywhere for a "long" time then we are good to go.

[Grotax]

Ah apparently PHP curl extension can tell you which capabilities are enabled, maybe we can simply ask the system what it supports and base the supported compression on this.

That way we also catch strange cases where people run Nextcloud on openwrt or something else that is small and limited on purpose.