Allow whitelisting some keys for idempotency check

Recently we have had the try-runtime tool fail in tanssi because of a state mismatch when running the idempotent check. The cause was 2 keys in pallet system: events and eventsCount, which changed because one of our migrations emitted an event to signal the start of the migration. Since events don't really affect chain state, we would like to ignore them.

Not sure how to implement this, we could hardcode some known keys and ignore them in the state diff comparison, or simply allow users to pass some flag with a list of keys to ignore.

Issue 提到了 try-runtime 工具、它的幂等性检查以及 state diff 比较，但没有指出文件或测试。首先定位幂等性检查的实现，以及 state keys 的比较方式；当用户可以配置忽略 events 和 eventsCount 等 keys，且不会掩盖其他 mismatches 时，即表示完成。