Support new XRay API configured sampling rules

[devinsba]

AWS just announce support for an API/UI to configure sampling rules. We should implement this feature to make use of brave more seamless in an XRay world.

Depends on existence of reservoir sampler: https://github.com/openzipkin/brave/issues/705

https://docs.aws.amazon.com/xray/latest/devguide/xray-console-sampling.html