Sidecar: refactor the `config` module

[merklefruit]

The config.rs file is getting crowded already. It's a good time to turn it into a directory with:

• mod.rs: same content as current config.rs
• chain.rs: Chain configs only

_Originally posted by @merklefruit in https://github.com/chainbound/bolt/pull/104#discussion_r1664115958_

[merklefruit]

closed in #108