Expose Kademlia K_VALUE as a configurable parameter

[noamsp-starkware]

Description

The Kademlia implementation in rust-libp2p currently uses a fixed K_VALUE which is not configurable by users.

When running a closed network of ~100 nodes, I observe cases where peers are unable to reliably discover or locate each other, despite the network being transitively connected and all peers being reachable.

Motivation

K_VALUE is a core parameter that impacts routing table density and lookup convergence. A fixed value limits the ability to adapt Kademlia behavior to smaller or private networks. Exposing this parameter would allow tuning without changing the default behavior.

Current Implementation

K_VALUE is a const.

Are you planning to do it yourself in a pull request?

Yes

[elenaf9]

Related: #5501 and #5875.
Does the discussion on those issues solve your issue?

[noamsp-starkware]

Nope.
Being able to configure the number of closest peers returned by a query would be very helpful in my case.
Here is the PR associated with my proposed fix: #6255