Internal: adjusted BroadcastShardOperationResponse subclasses visibility

Adjusted `BroadcastShardOperationResponse` subclasses visibility to package private when possible.
Also replaced `int`,`String` pair with `ShardId` that holds the same info and serializes it the same way.
Replaced shardId and index getters in `BroadcastOperationRequest` with a single `ShardId` getter that allows us to reuse the same object in responses.