Add child-bounties pallet. (#10309)

* Extract child-bounties as a separate pallet.

* Initial tests added.

* More tests.

* Tests complete. Fixed curator fee issue.

* Fixed comments.

* Added benchmarks for child-bounties pallet.

* Added weights.

* Fixed formatting.

* Fixed comments.

* Re-run benchmarks for bounties pallet.

* Make cargo fmt happy again

* add max encoded len

* use event structs

* fmt

* fix compile

* Addressed review comments.

* Use config type instead of const in benchmarking.

* Addressed more review comments.

* Use ensure_can_withdraw instead of just checking min balance.

* fmt.

* Introduce ChildBountyCuratorDepositBase to avoid zero curator deposits for child bounties.

* Fix unassign curator logic for child-bounties.

* Added more tests for unassign curator.

* Reduce bounty description max length in node runtime.

* Updated weights for child bounties pallet.

* reduce indentation of unassign_curator

* more indentation reduction

* deduplicate slashing arms

* reintroduce ensure check

* add assertion to check that bad unassign origin fails

* formatting

* Updated comments.

Co-authored-by: Ricardo Rius <[email protected]>
Co-authored-by: Shawn Tabrizi <[email protected]>
Co-authored-by: Alexander Popiak <[email protected]>