Add p50 and p95 of memory/CPU usage to benchmark summary script.

[aemous]

Description of changes:

• Include p50 and p95 of memory and CPU usage in summary, over all samples.

Description of tests:

• Manually ran the summarize script on benchmark results with JSON output format. Confirmed that the results appear numerically sound. One example output is shown below, which summarizes a benchmark of an execution of AWS CLI v2 sync upload.

{
  "total_time": 4.69900918006897,
  "std_dev_total_time": 0.0,
  "max_memory": 93241344.0,
  "std_dev_max_memory": 0.0,
  "p95_memory": 93241344.0,
  "std_dev_p95_memory": 0.0,
  "p50_memory": 93241344.0,
  "std_dev_p50_memory": 0.0,
  "average_memory": 88681056.26158038,
  "std_dev_average_memory": 0.0,
  "std_dev_max_cpu": 0.0,
  "max_cpu": 2.8,
  "p95_cpu": 2.2,
  "std_dev_p95_cpu": 0.0,
  "p50_cpu": 0.0,
  "std_dev_p50_cpu": 0.0,
  "average_cpu": 0.21771117166212528,
  "std_dev_average_cpu": 0.0
}