feat: output sample config

# Rationale

erigon supports a lot of options, and editing a commented out config, enabling what you need, might be a bit easier on the user than constantly referencing the help page.

i suggest we add a command that outputs a sample config in any format erigon supports (yaml, toml).

# Implementation

i made a sample script here that converts the `--help` command to a yaml config. https://gist.github.com/banteg/9427a4830be5a55f821064cdb02220d1