Expose client load balancing policy via command line

[CAFxX]

Is your feature request related to a problem? Please describe.
We need to use round-robin client load balancing when performing load tests. Without this we can't test deployments that scale out and that are not fronted by a load balancer (most of our services fall in this category).

Describe the solution you'd like
MVP: add a command-line option to opt-in to round-robin load balancing.

Ideally: add command-line options to set DefaultServiceConfig and DisableServiceConfig

Describe alternatives you've considered
N/A

Additional context
#242 added support for this in the runner, but did not expose a way to enable it via command line