Implement `/pallets/{palletId}/storage/{storageItemId}` (#278)

* Replace test response

* Get test to pass

* Update fields to null

* Include metadata in response

* Save progress

* Update openapi

* Update README

* Small updates

* More small changes

* Apply suggestions from code review

Co-authored-by: David <[email protected]>

* Improve comments and error messages

* Update err message

* Add example to docs; Refactor code; Documentation as string

* docs corection

* build docs

* Small changes;

* Add pallet index

* Update openapi specs

Co-authored-by: David <[email protected]>