[Proposal] Add envoyproxy/nighthawk's client as a load generator

Inspired by https://github.com/envoyproxy/envoy/issues/13412, I took a look at this repo, and at the surface
it seems to me that it would take a relatively small effort to introduce a wrapper for using Nighthawk's
load gen client [1].

If there's interest in a pull request, I'd be up for contributing this.

Nighthawk's offers some options and knobs that may be nice to have (for example explicit connection re-use strategies, open-loop mode, a gRPC service for remote load test execution).

[1] https://github.com/envoyproxy/nighthawk