Expose Kademlia K_VALUE as a configurable parameter

### 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