Add benchmarks/vegeta: HTTP load testing tool and library

Vegeta is a versatile HTTP load testing tool built out of a need to drill HTTP
services with a constant request rate.

WWW: https://github.com/tsenart/vegeta

PR: 222830
Submitted by: Vinícius Zavam <egypcio@googlemail.com>
Reviewed by: mat, swills
Differential Revision: https://reviews.freebsd.org/D12606