Implement `cksum -b`

[sylvestre]

According to GNU:

       -b, --base64
              emit base64-encoded digests, not hexadecimal

Tested in GNU with tests/cksum/cksum-a.sh
Source:
https://github.com/uutils/coreutils/blob/main/src/uu/cksum/src/cksum.rs

[sylvestre]

some work started here: #5122