Allow configuration of server-side request latency distributions

To test various behaviors for [certain Envoy filters](https://github.com/envoyproxy/envoy/issues/7789), it would be very handy to be able to specify request latencies with a weighted probability. This would allow us to observe the effect of tail latencies on filter behavior.

Something like this should work:
`--latency_distribution=zoned:3/1ms:1/4ms:1/20ms`

That would translate into:
- 3/5 of latencies should be 1ms
- 1/5 of latencies should be 4ms
- 1/5 of latencies should be 20ms