feat: add support for passing "all" arg to snap.logs

[PietroPasotti]

the snap logs command supports an all parameter for the -n flag.

# from the command's own --help:
[logs command options]
          --abs-time   Display absolute times (in RFC 3339 format). Otherwise, display relative times up to 60 days, then YYYY-MM-DD.
      -n=              Show only the given number of lines, or 'all'. (default: 10)
      -f               Wait for new lines and print them as they come in.

This PR adds support for that (in the form of a type hint correction, the implementation is unchanged).

[james-garner-canonical]

Looks good to me, please bump LIBPATCH if you want to have this fix released after merging.

[benhoyt]

@james-garner-canonical Can we close this and move the PR to https://github.com/canonical/charmlibs/tree/main/snap?

[james-garner-canonical]

Closing in favour of canonical/charmlibs#250