Fixed panic if malformed 3rd party url is specified

[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?

Fix the panic reported in #2786

What is the current behavior?

The arduino-cli panics by reproducing the steps described in #2786

What is the new behavior?

$ arduino-cli update-index
Error initializing instance: Invalid additional URL: parse "foo=https://arduino.esp8266.com/stable/package_esp8266com_index.json": first path segment in URL cannot contain colon
Downloading index: package_index.tar.bz2 downloaded                                                                                                                           
Downloading index: foo=https://espressif.github.io/arduino-esp32/package_esp32_index.json Unable to parse URL: parse "foo=https://espressif.github.io/arduino-esp32/package_esp32_index.json": first path segment in URL cannot contain colon
Some indexes could not be updated.
$

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

Other information

Fix #2786

[alessio-perugini]

changes LGTM. After we rebased this onto #2819 the CI should be 🟢

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.71%. Comparing base (f47399b) to head (e8e9a31).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
commands/instances.go	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2817      +/-   ##
==========================================
+ Coverage   67.62%   67.71%   +0.08%     
==========================================
  Files         238      238              
  Lines       22388    22388              
==========================================
+ Hits        15140    15159      +19     
+ Misses       6050     6034      -16     
+ Partials     1198     1195       -3     

Flag	Coverage Δ
unit	67.71% <77.77%> (+0.08%)	⬆️

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