Support more than `SLOTS_PER_HISTORICAL_ROOT` states when fetching state by root

[rkapka]

🚀 Feature Request

Description

Several Beacon API endpoints include a {state_id} parameter, which can be substituted with a state root. This is limited to the last SLOTS_PER_HISTORICAL_ROOT state roots (8192 on Mainnet). This is because there is no easy way to fetch an arbitrary state by root from the database, and we keep the latest SLOTS_PER_HISTORICAL_ROOT state and block roots in memory, which allows us to do this:

for i, root := range headState.StateRoots() {
	if bytes.Equal(root, stateRoot) { // stateRoot is the requested root
		blockRoot := headState.BlockRoots()[i]
		return p.StateGenService.StateByRoot(ctx, bytesutil.ToBytes32(blockRoot))
	}
}

Describe the solution you'd like

It would be desirable to be able to use an arbitrary state root for the {state_id} parameter. One possible solution is to create a state_root -> block_root mapping in the database. This way we could get the block root efficiently and then use StateGenService to fetch the entire state.

[int88]

@rkapka I'd like to implement if no one is working on this :)

[int88]

After some research, I found maybe it's good if we could extend StateSummary to make it include state_root. So we could easily find block_root by state_root.

But the problem is we should keep compatible with the StateSummary items that already in DB.

WDYT? @rkapka