modify the add-chain tool / flow to allow chains to opt in to receive hardforks

[geoknee]

We should make it easy for chains to opt into receiving hardforks even if they are not fully standard yet, I think that will tend to increase the usage of the --network flags. At the moment, they are automatically opted in when they promote to a standard chain, but we should explicitly document that they can opt in whenever they like.

We could have a flag in the .env file to declare opt-in, and the add-chain tool checks for that and sets superchain_time accordingly. See https://github.com/ethereum-optimism/superchain-registry/blob/main/docs/hardfork-activation-inheritance.md