[lab2] Update the exercises

We'll rework this lab for next year. This is a great proposal from @dorinel.filip:

TODO:

• Remove the code snippets for byte stuffing, this should be the main task of the lab.
• Remove the metrics part of the lab.
• Remove everything related to source/destination from the exercises.

New Tasks:

1. Send a string of different sizes.
2. Send a file from disk.

Everyone, feel free to discuss this here.