Option to use random user agent

[htrgouvea]

Context:

During the fuzzing process, many requests are sent, and web applications may have rate-limit controls that block or prevent fuzzing from continuing.
Some applications may also behave differently depending on the User-Agent sent. Dynamically changing the User-Agent during fuzzing can make it harder for applications to identify the fuzzing process while helping to identify specific behaviors or bypass restrictions.

What is needed to do:

Add an option (e.g., --random-agent) to use random User-Agents per request.
Implement a predefined list of User-Agents and allow customization via an external file.
Ensure the feature is compatible with other command-line options.
Update the documentation and create automated tests for the functionality.

Definition of done:

The --random-agent option is functional, uses random User-Agents, and is configurable.
Automated tests confirm the expected behavior.
The documentation is updated with clear examples.