add "config get" command to print settings values

[ardnew]

Please check if the PR fulfills these requirements

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• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Feature addition

What is the current behavior?

There are config commands to add/set individual settings, but retrieving settings requires the user to dump all settings and parse the output with either JSON/YAML to retrieve an individual.

What is the new behavior?

With the addition of a config get command, the user can retrieve individual configuration settings without having to parse JSON/YAML.

Does this PR introduce a breaking change, and is titled accordingly?

Not a breaking change

Other information

None

[MatteoPologruto]

Hello @ardnew! Thanks for your contribution.
Is this PR ready to be reviewed? If so, can you please rebase your branch onto arduino:master to include the latest commits and resolve any conflicts?
If you have any questions, please feel free to ask!

[ardnew]

@MatteoPologruto Will try to rebase this PR very soon to fix the conflicts.

Also, there were 2 items in the PR template that had not been completed (see below), which is why this was originally posted as a draft. Not sure if updating the docs is essential in this case or if I can submit the PR without it (after resolving conflicts)?

Checklist item	Status
Docs have been added / updated (for bug fixes / features)	Not sure which sections are mandatory to update. Is this at my discretion?
configuration.schema.json updated if new parameters are added.	Will not complete (no changes required)

[MatteoPologruto]

@ardnew don't worry, updating the docs is not needed in this case. The items you checked are the mandatory ones for this kind of contribution. I was asking if this was still a draft because I did not know if you were planning to further modify this PR. Since this is not the case, I will proceed with my review.

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (2b2394e) 68.95% compared to head (2942031) 68.97%.
Report is 3 commits behind head on master.

Files	Patch %	Lines
internal/cli/config/get.go	68.42%	10 Missing and 2 partials ⚠️
commands/upload/upload.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2307      +/-   ##
==========================================
+ Coverage   68.95%   68.97%   +0.01%     
==========================================
  Files         204      204              
  Lines       20535    20449      -86     
==========================================
- Hits        14160    14104      -56     
+ Misses       5220     5193      -27     
+ Partials     1155     1152       -3     

Flag	Coverage Δ
unit	68.97% <67.50%> (+0.01%)	⬆️

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[MatteoPologruto]

@ardnew code LGTM 🎉
You only need to update the tests according to the changes you made. After that, I'd say we are ready to merge!

[MatteoPologruto]

Great job! Thanks for your contribution @ardnew 🎉