feat: Introduce Importable/Exportable Storage traits.

[jsantell]

No description provided.

[jsantell]

• ImportStorage and ExportStorage traits for storages to pipe entries to another. A Storage is exportable if <S as Storage>::KeyValueStore: IterableStore. Any storage with a writable store <S as Storage>::KeyValueStore: Store can import. MemoryStorage, IndexedDbStorage, and SledStorage can be exported.
• A few generic storage operators: OpenStorage, RenameStorage, RemoveStorage. A generic open utility is valuable in tests alone, and other functions generic over storage, the others only used for migrating. A trait FsBackedStorage can be implemented, providing a blanket implementation to these operators.
• While IndexedDbStorage is exportable, a storage rename (hard copy) isn't ideal. While there are tests for exporting indexedDB, not yet handling migrating on the web without a destination format in mind (other than MemoryStore)
• The remaining bits are probing existing storage to learn the type/version if it exists to explicitly use DB types during migration.
• Names are murky for now; open to suggestions on FsBackedStorage, and any consistencies of storage-based traits; e.g. OpenStorage etc. (verb+Storage) seems to fit the bill in most cases (whereas Importable and Exportable kind of mean the same thing)

[jsantell]

Ready for review; would like to do the final bits in a follow up within noosphere where we determine if a migration is needed, what storage types are being involved, and any record keeping (db version/type), and actually enable the path