Logging setup and cli arg handling could be cleaner

Logging and CLI arg handling is a bit hacky.

For instance:
- some vars defined here and reused: https://github.com/flashbots/relayscan/blob/3a3dcb24167078f66952ad41034124360ba6bbec/cmd/service/service.go#L10
- logging actually instantiated here: https://github.com/flashbots/relayscan/blob/3a3dcb24167078f66952ad41034124360ba6bbec/common/logging.go

Improvements:
- Switch to https://github.com/urfave/cli for CLI argument handling
- Switch to built-in logger (see also https://github.com/flashbots/go-template/blob/main/cmd/cli/main.go#L26)

Start with cmd/service/service.go for the current CLI variables and common/logging.go for logger initialization; compare the referenced go-template CLI entry point and urfave/cli documentation. Done means CLI argument handling and logging follow the requested approaches without changing the service's observable behavior.

Written by the indexing model from the issue text.