Cli changes to expose dry-run mode for prune commands

[adhulipa]

This is a WIP commit based off of the main moby engine issue

I'd still have to add some more unit tests, I spent a bulk of my time rebasing over the past changes (phew!)

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "30623-dry-run-prune" [email protected]:adhulipa/cli.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354236464
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[adhulipa]

Testing on my box is taking a while. And each time I discover a new ridiculous mistake on my part. Close to completion though.

[codecov-io]

Codecov Report

Merging #1239 into master will increase coverage by <.01%.
The diff coverage is 47.82%.

@@            Coverage Diff             @@
##           master    #1239      +/-   ##
==========================================
+ Coverage   56.72%   56.73%   +<.01%     
==========================================
  Files         310      310              
  Lines       21800    21827      +27     
==========================================
+ Hits        12367    12383      +16     
- Misses       8518     8529      +11     
  Partials      915      915

[adhulipa]

@silvin-lubecki @vdemeester
I was wondering if you could take a quick glance at my CR?