Add relay monitors

[ltitanb]

fix #29

• Add support for relay monitor, similar to https://github.com/flashbots/mev-boost/blob/1fed7bb5138d37ecdfb204895e0c5a2179d735f9/server/service.go#L210-L224
• Change urls from String to Url, this should avoid some annoying bugs like not being able to parse a trailing slash in the url
• Also added a validate() function in the configs, not used for now but we should add some runtime validation which can be caught at init

[David-Petrov]

LGTM! 👍