[breaking] Updated gRPC Platform API

[cmaglie]

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• The PR has no duplicates (please search among the Pull Requests
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• 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?

A refactoring/rationalization of the gRPC Platform API.
This PR is draft to explore the soundness of the proposed changes.

What is the current behavior?

What is the new behavior?

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

Yes.

Other information

[codecov]

Codecov Report

Attention: 53 lines in your changes are missing coverage. Please review.

Comparison is base (eb8f2f2) 63.55% compared to head (8528612) 63.85%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2357      +/-   ##
==========================================
+ Coverage   63.55%   63.85%   +0.30%     
==========================================
  Files         205      206       +1     
  Lines       19289    19422     +133     
==========================================
+ Hits        12259    12402     +143     
+ Misses       5936     5925      -11     
- Partials     1094     1095       +1     

Flag	Coverage Δ
unit	63.85% <85.07%> (+0.30%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
arduino/cores/packageindex/index.go	90.25% <100.00%> (-0.09%)	⬇️
arduino/cores/packagemanager/loader.go	64.55% <100.00%> (ø)
arduino/cores/packagemanager/profiles.go	58.69% <100.00%> (ø)
commands/board/listall.go	89.23% <100.00%> (-1.91%)	⬇️
commands/core.go	100.00% <100.00%> (ø)
commands/core/upgrade.go	81.81% <100.00%> (+1.17%)	⬆️
commands/daemon/daemon.go	26.86% <ø> (+0.25%)	⬆️
internal/cli/arguments/completion.go	89.56% <100.00%> (+0.18%)	⬆️
internal/cli/arguments/reference.go	100.00% <100.00%> (ø)
internal/cli/core/list.go	95.71% <100.00%> (+2.09%)	⬆️
... and 12 more

... and 2 files with indirect coverage changes

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