feat: support multi chain inside one actor

[yezizp2012]

I think there will still be bug. If we have one actor like:

Union
  Chain
  Chain

The fragmenter won't split two chains into different fragments due to the recursive algorithm. Currently we don't have stateless executors with multiple inputs (like lookup), but I think we will need to support multiple chain inside one actor in the future.

Originally posted by @skyzh in #2775 (comment)

[yezizp2012]

The reason for the current restriction on the existence of multiple chains in the actor is that the ddl status report is bound to the actor, so just change it to be bound to the chain executors or when all the chains in the actor have reported before the actor finish can be counted. This can be changed after @BugenZhao finishes this PR #2773 .

Originally posted by @yezizp2012 in #2775 (comment)

[yezizp2012]

duplicated with #6236 .