feat: write `checksum` in _last_checkpoint file

[sebastiantia]

When CheckpoinWriter.finalize() writes the _last_checkpoint file, additionally write the checksum of the last checkpoint JSON as an additional checksum field.

For more information on JSON checksums, refer to the delta protocol section: https://github.com/delta-io/delta/blob/master/PROTOCOL.md#json-checksum