Add support for custom flags on validator clients

[erl-100]

Added support for user to specify their own flags in .env to be passed to the validator client

This is consistent with the same feature in eth-docker

One use case for this is to be able to pass in the flag for a proposer config file with the file path, so that you can specify a per validator fee address

[OisinKyne]

Hi @erl-100, good to see you :) one thing to note, is in a distributed validator, your fee recipient is set at the charon level, individual VCs don't get to decide it. (we're working on making the fee recipient addresses easier to update, but that still will be at the charon level). Other than fee recipients, do you need this feature for something else? :)

[erl-100]

Hi @OisinKyne, you too.. Understood. I was using it exclusively for that so ok to close this PR from my point of view, and I will stick with the charon setting. Would an update require rerunning create cluster or a simple edit to all existing cluster-locks?