Make gRPC downloads cancelable by complying to `context.Context` done signaling.

[cmaglie]

Describe the request

A download is currently not cancelable. This is not a big problem in the standalone CLI app, since it can be forcibly terminated with a CTRL-C, it may be a bigger problem using gRPC because if a download-triggering call is closed or canceled, the underlying download is not stopped.

To achieve this feature two tasks are needed:

• adding the possibility to inject a context in the Downloader library.
• pass the actual context from the upper call levels to the download library.

Describe the current behavior

A download is not stopped if the gRPC call that caused it is canceled.

Arduino CLI version

nightly

Operating system

N/A

Operating system version

N/A

Additional context

No response

Issue checklist

• I searched for previous requests in the issue tracker
• I verified the feature was still missing when using the nightly build
• My request contains all necessary details