[Tracking Issue] Flat storage for resharding V3

[Trisfald]

Part of #11881

Goal

Support resharding V3 for Flat Storage. On shard layout, Flat Storage must be able to take a shard and split it into two children.
The entire process will happen in the background with no strict time requirement. Additional processing resources consumption should be minimal because, in the meantime, the node will continue to apply blocks and chunks.

Sub-tasks

• Implementation
  • Support for splitting a Flat storage shard
    • Handle all Flat storage key-value pairs
    • Possiblity to interrupt resharding
    • Background processing
    • Creating and apply deltas on children shards
    • Handle parent shard deletion
  • Add observability
  • Integration with ReshardingManager

• Tests

  • Unit tests
  • Integration tests
  • Test with forknet