Add async_compression::Level controls, default to Level::Default

[JonLatane]

This builds on #8 slightly and adds async_compression::Level controls to Compress as well as the Compression and CachedCompression fairings. It also effectively switches all of the above to default to Level::Default instead of Level::Best, as I found its worst-case (first load) to average-case response times to be better for at least the CPUs of the servers I'm using. Happy to update the default either way, though I would argue making the default Level::Default has some clear merits 😄

(I'd previously mentioned trying to do progressive caching in the background but the Rocket Body lifetime makes this problematic... This works well in the meantime though!)

[Ameobea]

Thank you so much for this!! It definitely shouldn't be compressing at max level by default for dynamic data; that was an oversight on my part.

The only thing I'd tweak with this is actually overriding async-compression's Level::Default level for brotli. Internally, the default compression maps to Brotli's "Best" level due to the way they wrap the internal brotli crate. It's an issue I hit with tower-http and I even up a PR to fix it there: tower-rs/tower-http#356

So anyway, I think that we should add some logic to map a compression level of Default to an explicitly specified value of 4 if the compression type is brotli. We can still expose it to the user as default, and we'll just swap the default to a 4 before passing it into async_compression.

Gzip is fine and defaults to a more reasonable value. It's much faster to compress than brotli as well.

Thanks again for this!

[Ameobea]

I went ahead and updated this PR with those changes. I'll tag this off as 0.5.0 and publish once it's merged.

Ty again for implementing this!