[!115] Implement a 'zfill' command inspired by pythons zfill

Merge branch 'feature/zfill-command' into 'master'

See merge request quality/dune-testtools!115

(cherry picked from commit 539c674d)

ecdad585 Implement a 'zfill' command inspired by pythons zfill 17dfe8f8 Add a unit test for zfill