feat: Add --filename-prefix argument

[randhana]

This pull request adds a new --filename-prefix argument to allow users to add a prefix to the output file names. This makes it easier to organize results when performing multiple searches.

This resolves issue #2461.

Example Usage

# Run Sherlock with a filename prefix
python3 -m sherlock_project user1 --filename-prefix my-test-

# Verify that the output file is created with the prefix
ls my-test-user1.txt