Enhancement: Support for specifying file size in `check --read-data-subset`

To check a subset of repository files, the `check --read-data-subset` command
used to support two ways to select a subset - A specific range of pack files, 
or random percentage of pack files. We have added a third method to select pack 
files - By specifying file size. This new option is available with the 'restic check' command.

https://github.com/restic/restic/issues/3490
https://github.com/restic/restic/pull/3548