Expose new libzim settings to handle memory consumption

[benoit74]

https://github.com/openzim/libzim/pull/950 moved some settings from env parameters to CPP exposed settings. This has been released in libzim 9.3.0.

https://github.com/openzim/libzim/pull/960, https://github.com/openzim/libzim/pull/974, https://github.com/openzim/libzim/pull/975 and https://github.com/openzim/libzim/pull/982 pushed this further by allowing to configure the memory we want to consume. This is planned to be released in libzim 9.4.0.

These settings should be exposed in zim-tools as well.

zim-tools 3.6.0 introduced a kind of regression on this aspect since environment variables available in the past (and used in our infra) are not here anymore, and there is no new way to do the same thing.

Companion issues at libkiwix/kiwix-tools are https://github.com/kiwix/libkiwix/issues/1199 and https://github.com/kiwix/kiwix-tools/issues/745