Enhancement: Support random subset by size in `check --read-data-subset`

The `--read-data-subset` option of the `check` command now supports a third way
of specifying the subset to check, namely `nS` where `n` is a size in bytes with
suffix `S` as k/K, m/M, g/G or t/T.

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