feat: add teardown executor

[uniqueg]

Consider adding an executor that runs a script that shreds any decrypted files in the created volume.

We'd probably also need to set the ignore_error flag to true to make sure the teardown executor is executed even if actual task executor fails.

On the other hand, if the decryption itself fails, we don't want to start the actual task executor (ignore_error=false), yet ideally we still want to run the shredding script, just to be sure that whatever was written to the decryption volume, if anything, is shredded.

So maybe the flow could be something like this

• Decryption executor -> SUCCESS -> Task executor -> SUCCESS -> Teardown executor executes shredder script -> EXIT
• Decryption executor -> SUCCESS -> Task executor -> FAILURE - ignore_error=true -> Teardown executor -> EXIT
• Decryption executor: PROBLEM - catch within executor - execute shredder script - return non-zero exit status -> FAILURE - ignore_error=false -> EXIT