fix: check free disk space and allow to specify a different download/extraction directory

[MatteoPologruto]

Motivation

The default download/extraction directory might be on a disk with insufficient space.

Change description

Two changes have been introduced:

• allow the user to set a different temporary directory in which the Debian image is downloaded and/or extracted, using the --temp-dir flag;
• check that the temporary directory is in a disk with enough free space before starting a download/extraction.

Additional Notes

Reviewer checklist

• PR addresses a single concern.
• PR title and description are properly filled.
• Changes will be merged in main.
• Changes are covered by tests.
• Logging is meaningful in case of troubleshooting.

Requested changes have been made. Thanks!

[lucarin91]

Given the urgent need for this feature, I think we should merge the PR in its current state, but I would suggest executing a refactoring of the code to make it more maintainable #29