Enhancement: Verify data integrity before upload

Hardware issues or a bug in restic could cause restic to create corrupted files
that were then uploaded to the repository. Detecting such corruption usually
required explicitly running the `check --read-data` command.

To prevent the upload of corrupted data to the repository, restic now
additionally verifies that files can be decoded and contain the correct data
beforehand. This increases the CPU usage during backups. If absolutely
necessary, you can disable the verification using the option
`--no-extra-verify`.

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