Add methods to PrefixIterator to support iterating from a specific key (#9313)

* Add methods to PrefixIterator to support iterating from a specific key

* Expose the decode functions used in iterators for storage maps

* Use associated decode function in tests

* Revert "Expose the decode functions used in iterators for storage maps"

This reverts commit 34f57d92db89646d0c98ea1880df58d58e523b09.

* Fix documentation for next_key

* Add API for iterating from a specified key for all storage map types

* Enhance pagination test

* Add API methods to storage map types

* Rename next_key to last_key

* Rename last_key to last_raw_key

* Specify that iteration starts after starting_raw_key

* Update documentation on iteration ordering

* Rename next_key to previous_key

* Enhance pagination unit test

* Create unit tests for all kinds of iter_from methods

* Define iter_from in terms of iter rather than vice versa

* Cargo fmt