Add zstd-pkg-config feature

[figsoda]

This is so consumers of zip can use the pkg-config feature of zstd without manually adding a zstd dependency

[Plecra]

I believe the intended use of cargo is for dependent crates to explicitly opt into zstd themselves - this allows us to potentially update to newer versions of zstd internally even if they remove pkg-config support. If you manually add a zstd dependency, cargo will automatically find the suitable version of zip to continue supporting pkg-config. Do you think I'm getting something wrong here?