Use unsigned int 32 to decode cemented offsets

The offsets of the cemented files are encoded as signed int32 but decoded with signed int31. It allows referencing data of a file of 1Gib max. This could be an issue when writing cycles containing many big blocks.

To avoid that, we propose to use unsigned int 32, allowing the handling of files up to 4Gib.

As a future work, we may want to use int64 to avoid such limitation.

Manually testing the MR

Should be transparent. Green CI.

Checklist

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