Allow auth in URL

[KaloyanTanev]

🎯 Problem to be solved

Currently we support BN headers, but they are general for all nodes. If users have different auth headers for different BNs (>99% of the cases), users can't really achieve that with the current implementation.

🛠️ Proposed solution

• Allow basic auth in URL user:password@url

[DiogoSantoss]

Resolved under ObolNetwork/go-eth2-client#13