Support custom http headers in inference requests

[achandrasekar]

Fixes #181

[k8s-ci-robot]

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[achandrasekar]

Tested with the below config

None of PyTorch, TensorFlow >= 2.0, or Flax have been found. Models won't be available and only tokenizers, configuration and file/data utilities can be used.
2025-08-21 06:07:16,957 - inference_perf.config - INFO - Using configuration from: config.yml
2025-08-21 06:07:16,964 - inference_perf.config - INFO - Benchmarking with the following config:

api:
  type: completion
  streaming: true
  headers:
    x-something-header: something-header-value
    x-something-else-header: something-else-header-value
data:
...

2025-08-21 06:07:16,965 - inference_perf.client.filestorage.local - INFO - Report files will be stored at: ./reports
2025-08-21 06:07:19,617 - inference_perf.client.modelserver.vllm_client - INFO - Inferred model google/gemma-3-27b-it
2025-08-21 06:07:21,886 - inference_perf.loadgen.load_generator - INFO - Stage 0 - run started
2025-08-21 06:07:23,122 - inference_perf.client.modelserver.vllm_client - INFO - Headers in request {'Content-Type': 'application/json', 'x-something-header': 'something-header-value', 'x-something-else-header': 'something-else-header-value'}
2025-08-21 06:07:24,191 - inference_perf.client.modelserver.vllm_client - INFO - Headers in request {'Content-Type': 'application/json', 'x-something-header': 'something-header-value', 'x-something-else-header': 'something-else-header-value'}
2025-08-21 06:07:26,727 - inference_perf.client.modelserver.vllm_client - INFO - Headers in request {'Content-Type': 'application/json', 'x-something-header': 'something-header-value', 'x-something-else-header': 'something-else-header-value'}

[jjk-g]

/lgtm