Add Qubes OS isolation provider

[apyrgio]

Add an isolation provider for Qubes OS. Currently, we have two isolation providers:

• Container: This is our main isolation provider, which is used in every platform. Depending on the platform type, it uses Docker or Podman.
• Dummy: This isolation provider is used in some of our tests, e.g., in Windows/MacOS platforms.

The Qubes OS isolation provider should handle the first and second stages of the conversion:

• Specify a Qubes RPC policy that will allow running a Qubes RPC service on a disposable VM.
• Spawn a disposable VM for the first stage of the conversion, and specify the Qubes service that will run.
• Send the file to the disposable VM in the same way that the TrustedPDF sends the file.
• Receive the pixels from the disposable VM in a secure fashion.
• Convert the pixels to PDF, without OCR or compression.