Add load-test-qps parameter for dynamic client ramp up

[OVI3D0]

Description

Adds a new --load-test-qps parameter for OSB. This parameter, when used with an appropriate test procedure such as:

{
  "name": "timed-mode-test-procedure",
  "schedule": [
    {
       "operation": "range",
       "warmup-time-period": {{ warmup_time | default(300) | tojson }},
       "ramp-up-time-period": {{ ramp_up_time | default(100) | tojson }},
       "time-period": {{ time_period | default(300) | tojson }},
       "target-throughput": {{ target_throughput | default(20) | tojson }},
       "clients": {{ search_clients | default(20) }}
    }
  ]
}

Overrides the target-throughput and client parameters, setting them to the integer value passed in by the user.
OSB then uses the ramp-up property introduced in #725 to ramp up the number of clients over time until the target QPS is hit.

Example command:
opensearch-benchmark execute-test --pipeline=benchmark-only --target-hosts=<cluster address> --workload=big5 --test-procedure=timed-mode-test-procedure --kill-running-processes --load-test-qps=30 --telemetry=node-states --telemetry-params="node-stats-include-indices-metrics:'search'"

Issues Resolved

#736

Testing

• New functionality includes testing

make it + make test

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