Add bcmcli support to nic plugin - #266

Summary

• Added full bcmcli support (Thor Ultra, Thor3) with auto-detection, 36 per-device read-only commands, and dedicated parsers for device list and QoS
  output
• Added 35 new niccli per-device commands using v234+ syntax
• Added broadcom_cli_override arg to force bcmcli or niccli and skip auto-detection
• The plugin defaults to bcmcli, but falls back to niccli if bcmcli is not present or no managed devices are detected.