Enhancement: Allow pack size customization

Restic now uses a target pack size of 16 MiB by default. It can be customized
using the `--pack-size size` option. Supported pack sizes range between 4 and
128 MiB.

It is possible to migrate an existing repository to _larger_ pack files using
`prune --repack-small`. This will rewrite every pack file which is
significantly smaller than the target size.

https://github.com/restic/restic/issues/2291
https://github.com/restic/restic/pull/3731
