Child Bounty Pallet: chain bounty index grouped by parent bounty

[muharem]

Part of #5500

Goal:

Index child bounties by their parent bounty, ensuring that their indexes are independent of child bounties from other parent bounties. This will allow for predictable indexes and the ability to batch creation and approval calls together;

Solution:

To implement isolated child bounty indexes per parent bounty, we will add a new storage item that tracks the child bounty count per parent bounty without decrementing it upon child bounty completion. Similar to the ParentChildBounties, except that it keeps count of active child bounties. The ChildBountyCount can be deprecated.

We will need to adjust the keys for the ChildBountyDescriptions and ChildrenCuratorFees storage items (StorageMap -> StorageDoubleMap), changing them from (ChildIndex) to (ParentIndex, ChildIndex). The ChildBounties storage item will remain unchanged.

The migration required to translate data for ChildBountyDescriptions and ChildrenCuratorFees storage items.