Support new use case: bootstrap of new streaming query with initial state

[HeartSaVioR]

See below: actual end user asked the use case on Spark user mailing list.
https://lists.apache.org/thread.html/68226abe4be5ec979eecc3e62e8c05b3036aa0876d555411436a35ab@%3Cuser.spark.apache.org%3E

Initial state can be either specific values or calculate from batch data.

The issue is, structured streaming doesn't have a concept of "initial state" - at least in HDFSBackedStateStore. If the version is 0, it doesn't load the state.

So either we should propose a change to Spark to allow initial state for version 0, or create a new checkpoint which has "batch 0" with empty offset/commit but initial state.

We could try latter (less coupled with upstream) and propose a change on upstream if it doesn't work.

[HeartSaVioR]

I can't think about the approach of creating offsets/commits for empty batch - offset implementations are vary for data sources. Only one I can think it's possible is let state implementation allows version 0 of state as initial state. Unlike other stuff, it would only work if the state implementation supports version 0, as "supporting version" is not defined in the interface.