Allowing routing based on payload contents

### What should we add or change to make your life better?
Allowing routing requests to certain services (or service instances) based on certain parts of the payload.

### Why is this important to you?
We are on the listening end of a notification pipeline, like a lot of notification pipelines the auth info is not specific to a certain shard and instead the shard information is put in the request itself. We would want certain shards to go to certain instances. This will require inspecting payload.