[BE] Provide script to run on console for bulk assignment

Purpose of this MR

• The current MR presents the Code Suggestions Bulk User Assignment rake task.
• The script accepts a CSV file path as a parameter and handles user assignments.
• This task is specifically designated for manual use and not automated or running within the GitLab codebase.

Local Setup and Validation Instructions

For details on setting up and validating the script locally, kindly document the file in the MR.

Related to #431619

ee/lib/duo_pro/bulk_user_assignment.rb

 # frozen_string_literal: true
 
-# Code Suggestions Bulk User Assignment
-# 1. Set the `file_path` variable to point to your CSV file, e.g., `file_path = 'users.csv'`.
+# Duo Pro Bulk User Assignment
+# 1. Set the `add_on_purchase` variable to point to your AddOnPurchase record
+# add_on_purchase = GitlabSubscriptions::AddOnPurchase.find_by(add_on: GitlabSubscriptions::AddOn.code_suggestions.last)
+# 2. Set the `usernames` variable to point to your usernames array
+# usernames = ["user1", "user2", "user3", "user4", "user5"]
 # 3. Execute the bulk assignment:
 #    ```ruby
-#    DuoPro::BulkUserAssignment.new(file_path).execute!
+#    DuoPro::BulkUserAssignment.new(usernames, add_on_purchase).execute!
 #    ```
 
 # Error Messages: