Add service plugin and support in process integration.

Signed-off-by: Lantao Liu <lantaol@google.com>

client_opts.go

@@ -24,6 +24,7 @@ import (
 
 type clientOpts struct {
 	defaultns   string
+	services    *services
 	dialOptions []grpc.DialOption
 }
 
@@ -49,6 +50,17 @@ func WithDialOpts(opts []grpc.DialOption) ClientOpt {
 	}
 }
 
+// WithServices sets services used by the client.
+func WithServices(opts ...ServicesOpt) ClientOpt {
+	return func(c *clientOpts) error {
+		c.services = &services{}
+		for _, o := range opts {
+			o(c.services)
+		}
+		return nil
+	}
+}
+
 // RemoteOpt allows the caller to set distribution options for a remote
 type RemoteOpt func(*Client, *RemoteContext) error