Download library index as a combined archive+signature

[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)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

What kind of change does this PR introduce?

Download library_index.json as a combined index+signature archive from the new URL: https://downloads.arduino.cc/libraries/library_index.tar.bz2

What is the current behavior?

Library index was downloaded separately from signature: this lead to sporadic incorrect signature verification.

What is the new behavior?

The index and signature are obtained from a single tar.bz2 archive (that also provide a bit of saving of the file size).

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

No

[codecov]

Codecov Report

Base: 36.67% // Head: 36.62% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (138f9f9) compared to base (d27ba3d).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1908      +/-   ##
==========================================
- Coverage   36.67%   36.62%   -0.05%     
==========================================
  Files         231      231              
  Lines       19684    19683       -1     
==========================================
- Hits         7219     7209      -10     
- Misses      11635    11646      +11     
+ Partials      830      828       -2     

Flag	Coverage Δ
unit	36.62% <100.00%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
commands/instances.go	39.64% <100.00%> (-0.16%)	⬇️
arduino/resources/index.go	40.20% <0.00%> (-12.38%)	⬇️
arduino/cores/packagemanager/package_manager.go	66.58% <0.00%> (+0.75%)	⬆️

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