Customizable request source without RPC

Issue

If a customer wants to connect to multiple clusters via the same load balancer instance, it may be difficult to do so with the #current request source implementation. A separate request source grpc server is required, and it introduces two extra connections between the test controller and the rpc Request Source and between the rpc Request Source and the Nighthawk Traffic Generator and is a potential bottleneck. Looking for a solution that allows request sources to be more easily customizable and is internal to the Nighthawk Traffic generator.

Potential solution

Refactor the nighthawk request source implementation to support a plugin infrastructure. This way a request source can be implemented internally to nighthawk and chosen at runtime. We should keep backwards compatibility with gRPC request source instances.