[FLINK-35660] Adding parent-child shard ordering for DDB Streams source

[gguptp]

Purpose of the change

• Implements parent-child shard ordering in DDB Streams source
• Source enumerator will only assign splits that correspond to a shard when parent shard is either finished, expired or doesn't exist
• Source enumerator also discovers inconsistencies in the shard graph due to describestream and fixes it

Verifying this change

Added unit tests

Significant changes

(Please check any boxes [x] if the answer is "yes". You can first publish the PR and check them afterwards, for convenience.)

• [ ] Dependencies have been added or upgraded
• [ ] Public API has been changed (Public API is any class annotated with @Public(Evolving))
• [x] Serializers have been changed
• [x] New feature has been introduced
  • If yes, how is this documented? (not applicable / docs / JavaDocs / not documented)