dep(maili): Dependency set in RollupConfig

[clabby]

Overview

The dependency set for interop will be stored within the RollupConfig. The proposed schema from the Go team is something like:

dependencySets: {
 1000: [1,2,3,4], // timestamp to list of chain IDs
 3500: [1,2,3,4,5],
 7000: [1,2,3]
}

The FPP will need this field added to be able to reference the canonical dependency set of the chain to perform validation on the SuperRoot it receives.

[clabby]

I don't like the proposed schema. It makes no sense to have to define dependency set timestamps separate from hardfork timestamps imo.

We still don't have a great idea of how the dependency set will be updated - on-chain, or through the client software. If through the client software, I think I'd rather pair dependency sets with forks, rather than timestamps (just because there's more room for error if the timestamps are set in 2 places.) If not through the client software, we have to do something different & this ticket can be closed.

[ajsutton]

We can't directly combine the dependency set updates and the hard forks because while dep set updates are a hard fork, they are also unique to an interop cluster. So you can't say that Holocene added Base to the dependency set because on a different cluster that may not be the case. So somewhere in the rollup you need a mapping from activation timestamp to the list of chains being added to the dependency set (removal is not possible). The activation timestamp might be the same as a named hard fork like holocene, but it doesn't have to be (it is still a hard fork though). There's a bunch of ways you might try and express that in JSON and I don't really mind which we go with, but the simple version above seems pretty reasonable to me.