v4.2.0

• feat: support distributed load testing on multi-machines
• feat: support run/boom/convert curl command(s)
• feat: add ping/dns/traceroute/curl sub commands
• feat: improve builtin uploading feature, support @ indicator and inferring MIME type
• feat: hrp boom support setting duration of run time
• change: support omitting websocket url if not necessary
• change: support multiple websocket connections for each session
• fix: optimize websocket step initialization
• fix: reuse plugin instance if already initialized
• fix: deep copy api step to avoid data racing