Add the possibility to specify a custom command, checking its output.

[tcaiazzi]

It would be useful to extend the tool by allowing users to specify custom commands within their configurations. The tool should then run the custom command and check its output against expected results, providing feedback if the output does not match.

Proposed Feature:

• Add the ability for users to define a custom command in the exercise configuration file.
• The tool should execute the specified command during the checking process.
• Capture and compare the command's output to an expected value provided in the configuration.
• Provide feedback to the user if the output does not meet the expected result.

Benefits:

• This feature would provide more flexibility in creating tests for network exercises.
• It allows for additional checks based on the output of specific commands.