Added --jobs/-j flag to compile command

[cmaglie]

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
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• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Adds the --jobs/-j to the command line interface to allow fine-tuning the number of parallel threads used during compilation.
The gRPC API is unchanged because it already had this parameter available.

What is the current behavior?

Using Arduino CLI as a command line app, the number of parallel jobs was fixed to the number of available CPU cores in the system.

What is the new behavior?

The maximum number of cores can be tuned with --jobs/-j flag.

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

No

Other information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4a5585e) 0.00% compared to head (9cd0fe0) 68.96%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2526       +/-   ##
===========================================
+ Coverage        0   68.96%   +68.96%     
===========================================
  Files           0      204      +204     
  Lines           0    20523    +20523     
===========================================
+ Hits            0    14154    +14154     
- Misses          0     5216     +5216     
- Partials        0     1153     +1153     

Flag	Coverage Δ
unit	68.96% <100.00%> (?)

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