Add way to swap Flate2 feature zlib-ng => zlib-rs

## What is the problem your feature solves, or the need it fulfills?

Compiling Flate2 with the zlib-ng feature on Windows fails with the error:
error: could not find native static library 'zlibstatic-ngd'.
Switching to zlib-rs resolves this issue.

## Describe the solution you'd like

I'd like a feature or configuration option in Pingora that activates the zlib-rs feature in the flate2 crate instead of zlib-ng.

## Describe alternatives you've considered

- Using the 'miniz_oxide': default backend would work as well.
- Help me patch out 'zlib-ng': I swear I've tried but it won't work 😭, a less involved solution would be preferred.
- Manually building zlib-ng and linking with -L Rustflags: This is what I've been doing, but it's not ideal for CI or our cross platform builds.

## Additional Context

My primary concern is making the Windows build succeed. The zlib-ng feature in flate2 causes build failures due to missing native libraries. Switching to zlib-rs resolves this, and I'd love to see Pingora support that path more directly.