`bin/ec`: produce validator directories for lighthouse

[ralexstokes]

having produced (public_key, keystore, password) tuples for some number of validators, provide a command that writes these in the directory structure expected by a validator client

target lighthouse for now, for simplicity

[ralexstokes]

this may be better to just target this format and then use the lighthouse validator-manager import command in ops

https://github.com/sigp/lighthouse/blob/441fc1691b69f9edc4bbdc6665f3efab16265c9b/validator_manager/src/common.rs#L42-L50