[breaking] Refactor `DownloadProgress` gRPC message: more clear field meaning.

[cmaglie]

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Please check if the PR fulfills these requirements

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

• What kind of change does this PR introduce? Refactor the DownloadProgress messages.

• What is the current behavior? It's not very clear how the fields in the progress reports are populated.

• What is the new behavior? The new message structure should be much more clearer.

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

[codecov[bot]]

Codecov Report

Base: 36.58% // Head: 36.77% // Increases project coverage by +0.19% :tada:

Coverage data is based on head (c28cfd3) compared to base (5730e2e). Patch coverage: 58.06% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1875      +/-   ##
==========================================
+ Coverage   36.58%   36.77%   +0.19%     
==========================================
  Files         231      231              
  Lines       19696    19685      -11     
==========================================
+ Hits         7206     7240      +34     
+ Misses      11659    11614      -45     
  Partials      831      831              

Flag	Coverage Δ
unit	36.77% <58.06%> (+0.19%)	:arrow_up:

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

Impacted Files	Coverage Δ
cli/core/search.go	0.00% <0.00%> (ø)
cli/core/update_index.go	0.00% <0.00%> (ø)
cli/update/update.go	0.00% <0.00%> (ø)
commands/daemon/daemon.go	0.00% <0.00%> (ø)
commands/instances.go	39.79% <15.38%> (+0.70%)	:arrow_up:
arduino/httpclient/httpclient.go	60.71% <54.54%> (-6.56%)	:arrow_down:
cli/output/rpc_progress.go	80.88% <88.88%> (+12.13%)	:arrow_up:
arduino/cores/packagemanager/download.go	29.48% <100.00%> (+6.59%)	:arrow_up:
arduino/resources/download.go	62.50% <100.00%> (-5.36%)	:arrow_down:
cli/instance/instance.go	57.28% <100.00%> (-0.42%)	:arrow_down:
... and 8 more

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[kittaakos]

@cmaglie, could you please merge this PR if it's ready? Thank you!