feat: saveWhen in HydratedBloc.

[xuxiaocheng0201]

Description

Currently, whenever the state changes, stoarge.write is called. code
(Although it doesn't write when toJson returns null, I actually don't know when to return null.)
But sometimes, certain fields in the state are not stored and these fields change very frequently, such as loading progress. In this case, saving every state change would result in a significant amount of unnecessary duplicate writes.

Desired Solution

A saveWhen method should be provided. Such as buildWhen in BlocBuilder.

Alternatives Considered

Or provide both currentState and nextState parameters in toJson method.