cmd/mev-boost: add urfave as cli framework

[MariusVanDerWijden]

📝 Summary

This PR adds urfave/cli as a cli framework and reworks some of the logic in the main package.
It also gets rid of the weird redirections into the cli package

⛱ Motivation and Context

Cleanup, adds a cli framework which makes adding flags easier in the future

[MariusVanDerWijden]

closes #640

[metachris]

Cool, thanks! A few quick notes

• The "weird indirection" was to allow go install github.com/flashbots/mev-boost (it won't work without a root main.go file i think)
• I like urfave a lot, and the cli flags have grown significantly and unpleasantly in mev-boost. But is the switch to urfave backwards-compatible, or would it break? This seems to be a major concern, if the upgrade would force all proposers to update their cli flags.

[metachris]

@MariusVanDerWijden can you please add the root main.go file back, so go install github.com/flashbots/mev-boost works?