Installer: install board cores before uploading

When uploading a sketch with the Installer for a specific board, if the corresponding Arduino "core" is not installed on the user's machine, the process fails.
Usually this is not happening for advanced Arduino users, since very likely they have the Arduino IDE with a few cores already installed.
But on a new computer, chances are that the user never installed the Arduino IDE or that specific core.

Suggested solution: list the supported cores (via their fqbn) and use the embedded `arduino-cli` to install all supported cores.