Switch grpc clients to use configurable service config

[sluongng]

https://grpc.io/docs/guides/service-config/#behavior-controlled-by-the-service-config

https://github.com/grpc/grpc-proto/blob/master/grpc/service_config/service_config.proto

This way, if a user wants, they can override the default JSON config with their own to implement different timeout values for different RPCs.

The out-of-the-box retry and hedging implementations are also nice to have.

[sluongng]

SISO (chromium build tool) has a really good example of a service config for remote API would look like https://source.chromium.org/chromium/infra/infra_superproject/+/main:infra/go/src/infra/build/siso/reapi/reapi.go;l=214-275;drc=26e0caea23e77ba4c4d7be55e5d93781321efd20