Add basic intoto apt transport method (http transport relay)

[lukpueh]

Fixes issue #: None

Description of the changes being introduced by the pull request: Basic in-toto apt transport that forwards messages from apt to the builtin http transport and vice-versa, providing stubs to intercept certain messages to perform in-toto verification for the debian package to be installed. See README.md and the extensive code docs and comments in intoto.py for further details.

The commits of this branch are squashed commit, the corresponding WIP-trial-and-error git history is available in lukpueh/apt-transport-in-toto@basic-transport-wip.

Note, the git history contains a lot of trial and error

Please verify and check that the pull request fulfills the following requirements:

• [x] The code follows the Code Style Guidelines
• [ ] Tests have been added for the bug fix or new feature
• [X] Docs have been added for the bug fix or new feature