Add gRPC function to save the current config to the default path

[4ntoine]

Bug Report

Current behavior

Currently SetValue does not save the settings to disk. It get's cleared when daemon restarted.

Write request requires file path.

How can one save the settings to default path?

Expected behavior

Write accepts no path specified and saves it to the default path.

Environment

• CLI version (output of arduino-cli version): 0.19.0
• OS and platform: macOS 10.14.6

Additional context