Allow custom load balancing for gRPC [agent]

[guanw]

Requirement - what kind of business use case are you trying to solve?

Allow external discovery service to integrate with grpc by creating a grpc resolver and using notifier.

Problem - what in Jaeger blocks you from solving the requirement?

None

Proposal - what do you suggest to solve the problem or improve the existing situation?

Add a resolver component that implements resolver.Resolver interface and integrate with discovery.notifier interface.

Any open questions to address

None