feat(kad): make Distance private field public

[maqi]

Description

make Distance private field (U256) public

So that some network density in Distance can be calculated as below:

let density = U256::MAX / U256::from(estimated_network_size);
let density_distance = Distance(estimated_distance);

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[maqi]

Hi, @jxs @guillaumemichel ,

May this PR to be reviewed and approved to merge soon?
thx a lot.
We have project depends on this.

btw. the failing CI is not related to this PR.
seems PR #5702 tried to mute it, however is not preferred ?

[guillaumemichel]

Makes sense to me, thanks @maqi!

The failing CI check isn't a problem, we can merge anyway.

[maqi]

Hi, @guillaumemichel ,

thx a lot for the prompt approval.

[dariusc93]

LGTM