fix(cli): Convert policy set --manual to use a tri-state

[PhracturedBlue]

As per issue #3344, If Manual is set to true in a more-general policy, it cannot be reset to false in a lower policy. This is due to manual being a bool, and there being no way to distinguish between 'inherit' and 'false'

This PR converts Manual into an OptionalBool so that it can hold values of true, false, and inherit, and can thus override both from false->true and from true->false

There is an included test to validate as well.

This PR includes #3348 as inheritance cannot be properly tested until that issue is addressed. I recommend committing #3348 1st, and then this PR

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (814738c) 75.88% compared to head (de49f42) 75.83%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3349      +/-   ##
==========================================
- Coverage   75.88%   75.83%   -0.05%     
==========================================
  Files         462      462              
  Lines       37082    37087       +5     
==========================================
- Hits        28138    28125      -13     
- Misses       7020     7032      +12     
- Partials     1924     1930       +6     

Files	Coverage Δ
cli/command_policy_set_scheduling.go	86.55% <100.00%> (+0.11%)	⬆️
cli/command_policy_show.go	60.00% <100.00%> (+0.44%)	⬆️
snapshot/policy/scheduling_policy.go	85.07% <80.00%> (ø)

... and 5 files with indirect coverage changes

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[lupusA]

This PR will close #3344