AggLayer: Store a claimed global index hash chain

[mmagician]

Separately, we still have to store a hash chain of the GERs as a double word.

Originally posted by @mmagician in #2341

[Fumuran]

Previously there was a confusion between the global index root and claimed global index, in fact we should store the claimed global index hash chain (link in the description correctly points to the claimedGlobalIndexHashChain value in the Solanum code).

Side note: is it correct to name this value a "CGI hash chain" instead of "CGI chain hash"? It is named that way in the Solidity code, but the final value, which should be stored in memory (or elsewhere) is a hash, not a chain. This value represents the sequential hash of hashes, so in some way it is indeed hash chain, and the resulting value is just the head of this chain, but still IMO it is more correct to name it chain hash. @mmagician WDYT?

[mmagician]

Thank you for updating our understanding here. Yes the suggested naming sounds good (I assume this is what they also use in Solidity which should be the ground truth for the line of work)