expose balance-change events in a block

[bowenwang1996]

For a given block, we can query for each account the events that happen in that block, but there is no way to query all the events that change account balances for all the accounts that are touched in the block.

[frol]

Since #2253 there is a new RPC endpoint to query a list of accounts with a type of the state change (e.g. account_touched) touched within a given block.

Since #2148 the changes RPC exposes detailed information about the state change (a cause and a new value).

#2264 will allow querying RPC for state changes of the same type (e.g. account_changes) for a list of account_ids.

@bowenwang1996 Do those PRs resolve this issue? Is there anything extra we want to expose?

[bowenwang1996]

I believe they are sufficient.