Allow platforms without fixed version in profiles.

[cmaglie]

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• Tests for the changes have been added (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?

The idea is to allow in the sketch.yml platforms without a specific version, like the following giga_any:

profiles:
  giga:
    fqbn: arduino:mbed_giga:giga
    platforms:
      - platform: arduino:mbed_giga (4.3.1)

  giga_any:
    fqbn: arduino:mbed_giga:giga
    platforms:
      - platform: arduino:mbed_giga      # this does not specify a version
      # no need to specify index_url either

default_profile: giga_any

In this case, the Arduino CLI will not download the platform to the internal space, but it will use the globally installed platform.

What is the current behavior?

You must specify a version in the platform directive of the sketch.yml

What is the new behavior?

You can omit a version in the platform directive of the sketch.yml, in this case the Arduino CLI will use the globally installed platform.

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

No

Other information

[codecov]

Codecov Report

Attention: Patch coverage is 68.96552% with 18 lines in your changes missing coverage. Please review.

Project coverage is 68.03%. Comparing base (3dca438) to head (863f46e).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/arduino/sketch/profiles.go	80.00%	5 Missing and 5 partials ⚠️
commands/instances.go	0.00%	4 Missing and 1 partial ⚠️
internal/arduino/cores/packagemanager/profiles.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2940      +/-   ##
==========================================
+ Coverage   67.96%   68.03%   +0.06%     
==========================================
  Files         238      238              
  Lines       22524    22567      +43     
==========================================
+ Hits        15309    15353      +44     
  Misses       6013     6013              
+ Partials     1202     1201       -1     

Flag	Coverage Δ
unit	68.03% <68.96%> (+0.06%)	⬆️

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