Add preference to set a custom update url

[AlbyIanna]

Motivation

We want to have a way to test the automatic update feature when downloading a build from a PR.

Change description

Add the possibility to change the URL the updater uses to know if there are updates and to download them. This way, anyone could start a local server that serves the files needed for the automatic update and fool the updater to make them think there is a new update to download.

Other information

This is necessary because a PR build of the Arduino IDE will generally be a newer version than the current latest version, so it's difficult to test before merging.

Reviewer checklist

• PR addresses a single concern.
• The PR has no duplicates (please search among the Pull Requests before creating one)
• PR title and description are properly filled.
• Docs have been added / updated (for bug fixes / features)

[AlbyIanna]

Here's some instructions to test this PR:

1. download the channel file. This differs based on the OS and the channel you're getting updates from, but now only the nightly channel is available:
   • MacOS
   • Windows
   • Linux
2. download the latest nightly from https://downloads.arduino.cc/arduino-ide/nightly/[NAME_OF_LATEST_BUILD] and replace NAME_OF_LATEST_BUILD with the value of the property path inside of your channel file (e.g.: https://downloads.arduino.cc/arduino-ide/nightly/arduino-ide_nightly-20220225_Windows_64bit.exe)
3. create a folder called test-updater
4. inside test-updater, create another folder called nightly
5. move the channel file and the build file you've just downloaded inside the nightly folder
6. serve content of test updater with whater local server you want. For example

cd test-updater    
python3 -m http.server

6. Download and open the PR build of the Arduino IDE you want to test
7. Open the User Preferences from the command palette (CTRL/CMD + SHIFT + P)
8. Search for the "Update Channel" preference and set it to "nightly"
9. Search for the "Update Base URL" preference and set it to the ip address your local server is running on (e.g.: http://0.0.0.0:8000)
10. Close the Arduino IDE and reopen it. You should now see the update dialog

(Hint: after making your tests, make sure to reset the update URL to the default or your Arduino IDE will keep looking for updates from your custom URL)

[per1234]

I've been using this new feature a lot on Windows and Linux. It works perfectly and really improves the ability to test the update capability.

Thanks Alberto!