Support window_bits parameter for gzip filter
Support window_bits parameter for gzip filter

The window_bits parameter is exposed by Envoy (see [docs](https://www.envoyproxy.io/docs/envoy/latest/api-v2/config/filter/http/gzip/v2/gzip.proto)),
and according to our testing is one of the most important settings for
the resulting compressed size.

On averaging a few JS & CSS files, we get pretty consistently (to a few percent):
- gzip, no extra configuration: 100 (baseline)
- gzip, compression_level=BEST: ~99
- gzip, compression_level=BEST, window_size=15: ~80

So setting this gives us 20% extra compression!

The tradeoff in memory usage is documented by Envoy, whose documentation
I've lifted in this PR.

Background: https://github.com/envoyproxy/envoy/issues/8448

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