Enhancement: Add extra verification of data integrity before upload

Hardware issues, or a bug in restic or its dependencies, could previously cause
corruption in the files restic created and stored in the repository. Detecting
such corruption previously required explicitly running the `check --read-data`
or `check --read-data-subset` commands.

To further ensure data integrity, even in the case of hardware issues or
software bugs, restic now performs additional verification of the files about
to be uploaded to the repository.

These extra checks will increase CPU usage during backups. They can therefore,
if absolutely necessary, be disabled using the `--no-extra-verify` global
option. Please note that this should be combined with more active checking
using the previously mentioned check commands.

https://github.com/restic/restic/issues/4529
https://github.com/restic/restic/pull/4681
