fix: snap log num_lines can be 'all' as well as an int

[james-garner-canonical]

This PR ports the type annotation fix proposed in canonical/operator-libs-linux#166.

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Original description:

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).