chore: integrate discv5 config builder in networkconfig builder

[mattsse]

the separate setter with the closure is very inconvenient

https://github.com/paradigmxyz/reth/blob/cb0217bc51a8705145d573f672dff3c80ea30b82/crates/net/network/src/config.rs#L118-L118

we should get rid of that

but need some help with the last pieces @emhane