[TM-357] Partial UStore fill

Description

Add a version of fillUStore which can be used in the middle of migration.

One use case for it is in TBTC project - they have admin field which is present in V0 version of storage and thus fillUStore should not work there (it can migrate only from scratch). AFAIU fillUStore is still used there only because migrations are not yet fully type-safe - one can implement UStoreMigration V1.Storage V2.Storage and then apply it to migrate between two arbitrary storages; this is going to be fixed in 2nd MR of TM-337.

Related issue(s)

https://issues.serokell.io/issue/TM-357

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