Add -unique flag for filtering duplicate response sizes

Description

This PR adds a new -unique flag to ffuf that filters out responses with duplicate sizes, showing only the first occurrence of each unique response size. This feature helps reduce noise in the output and makes it easier to identify distinct responses during fuzzing.

Features

• New command line flag: -unique
• When enabled, only shows the first occurrence of responses with the same size
• Integrated with both normal and JSON output modes
• Thread-safe implementation for concurrent fuzzing

Implementation Details

• Added UniqueSizeFilter to handle response size filtering
• Integrated with existing output system to maintain compatibility
• Thread-safe using mutex to handle concurrent requests
• Zero performance impact when feature is disabled

Use Case

When fuzzing endpoints, responses with the same size often indicate similar or identical content. By showing only unique sizes, users can more easily identify distinct responses that might indicate successful fuzzing attempts.

Example usage:

ffuf -w wordlist.txt -u https://target/FUZZ -unique

Testing

• Tested with concurrent requests to ensure thread safety
• Verified behavior with both normal and JSON output modes
• Confirmed zero impact on performance when disabled
• Tested with various response sizes and content types

Documentation

• Added help text and usage information
• Updated README with new flag description
• This PR description highlights the key aspects of the feature, its implementation, and testing, making it clear to reviewers what the change does and why it's valuable.
• Would you like me to help you refine any part of this PR description or provide more specific technical details about the implementation?