Rewrite the update logic to download and replace the app bundle

[umbynos]

Rewrite the update logic (inside the GO executable) for macOS. This new logic will:

1. download the zipped bundle inside an OS temporary folder
2. unpack it inside an OS temporary folder
3. rename the current bundle to ArduinoCreateAgent-old.app
4. copy the new bundle from the temporary folder into the Applications folder
5. run the new agent (that has just been copied)
6. trash or delete ArduinoCreateAgent-old.app
7. exit the old executable (that now is running in memory only)

This is dependent from:

• The CI should upload a signed zip bundle to S3, for the new update logic #736
• The CI should upload a json with version and checksum to S3, for the new update logic #757

[umbynos]

If something goes wrong (eg not enough permissions) there should be a rollback procedure to restore the previous situation.