Mask api keys in genai_perf's logs

[e-dobrowolska]

Right now, genai_perf prints unmasked credentials in logs:

[2025-07-09 14:34:23] INFO     Running Perf Analyzer : 'perf_analyzer -m meta/llama-3.1-8b-instruct -async     subcommand.py:98
                               --stability-percentage 999 --request-count 10 -i http -u                                               
                               https://integrate.api.nvidia.com --concurrency-range 1 --service-kind openai                           
                               --endpoint v1/chat/completions -H Authorization: Bearer                                                
           printed here -->    <api_key_here> --input-data
                               ...[the rest of the comand]

This small PR changes the logging statement to mask the api keys (replace them with asterisks).
Example log after the change:

[2025-07-09 22:09:42] INFO     Running Perf Analyzer : 'perf_analyzer -m meta/llama-3.1-8b-instruct --async  subcommand.py:102
                               --stability-percentage 999 --request-count 5 -i http -u                                              
                               https://integrate.api.nvidia.com --concurrency-range 1 --service-kind openai                         
                               --endpoint v1/chat/completions --max-threads=1 -H Authorization: Bearer ********                     
                               --input-data                                                                                         
                               ...[the rest of the comand]