feat: 📊 write load test Summary data to file(s)

[cratelyn]

when a load test has concluded, a Syndicate returns a Summary. this structure contains data like the number of workers that were run, the number of workers that succeeded, and the duration of each worker task.

https://github.com/cratelyn/aquarius/blob/main/src/summary.rs#L16-L23

to facilitate in graphing with tools like, e.g. ggplot2, aquarius should provide a way to write the Summary data to a collection of files.