Add ability to save resources

[parafoxia]

Summary

This PR adds the ability to save attachments (and more generally, resources) to disk.

• Adds the Resource.save() method
• Adds tests for Resource.save() and Resource.read()

Checklist

• [x] I have run nox and all the pipelines have passed.
• [x] I have made unittests according to the code I have added/modified/deleted.

Related issues

None