GitLab

* Use downloader.SetDefaultConfig() to set user-agent for the arduino-cli

This change allows to not pass-trough the downloader configuration from
function to function everywhere (and sometime we forget to pass it for
example in the "core update-index" command).

* Add support for network proxy configuration

* Cosmetic: added blank space in configuration/defaults.go

* Update go-downloader to 1.2.0

* Refactored function to generate user-agent

* Added a TODO for missing proxy usage in board list API query

* Partially revert global network configuration.

It's better to read the network configuration before each http request so
in case it is changed using "Settings" functions in daemon mode the changes
are immediately applied.