feat: enable using the zlib-rs backend through flate2

[hasenbanck]

This backend is exposed since flate2 v1.0.29.

[hasenbanck]

• Build adn test fails, because the new optional feature would be require rustc 1.75 (zlib-rs), where as the pipeline currently uses 1.73.
• style_and_docs check fails because of files not touched by this PR.
• check_minimal_versions won't compile, because the test uses "cargo check --all-features", which will break flate2 (flate 2's CI does choose the backends selectively).

Not sure how to progress with this PR under such constraints.

[Pr0methean]

You'll probably need to increase the MSRV to 1.75 in Cargo.toml, and either (a) update the CI workflow to replace --all-features with all the maximal compatible sets of individual features, or (b) fix flate2 upstream so that it builds with --all-features.

[Pr0methean]

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