feat: Impl `/pallets/{pallets}/storage`; Impl index lookup for...

feat: Impl `/pallets/{pallets}/storage`; Impl index lookup for `/pallets/{palletId}/storage/{, storageItemId}` (#356)

* feat(endpoint): Add `pallets/{pallets}/storage`

* feat(endpoint): Add onlyIds query param to pallets storage

* feat(endpoint): Support index look up for pallets

* chore: Test `fetchStorage`

* chore: Test `fetchStorageItem`

* Apply suggestions from code review

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

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