Add ChildTriePrefixIterator and methods (#8478)

* Make use of PrefixIterator underneath Storage[Key]Iterator

* Add ChildTriePrefixIterator and methods

* Add documentation on ChilTriePrefixIterator fields

* Deprecate Storage[Key]Iterator API instead of removing them

* Allow fetching for the prefix as an option for ChildTriePrefixIterator

* Rename prefix_fetch to fetch_previous_key

* fix implementation + test

* make gitdiff better

* Add test for storage_iter and storage_key_iter

Co-authored-by: thiolliere <gui.thiolliere@gmail.com>