Support "keep" (cache) behavior when downloading tools

[Xayton]

The Arduino Cloud Agent can download tools, when requested by the front-end application. There are two ways to request tools download (using the WebSocket connection, or with a REST API endpoint).

With the #953 change, both cases use the same code, and in both cases the keep behavior is ignored. So every time a tool is requested, the agent will download it (even if it's already installed locally).

Analyze the existing code, the installed.json file and add again the functionality (eventually refactoring how it was working before).