fn: agent call overrider (#1080)

Similar to LB Agent call overrider, this PR adds Agent overrider
for Agents to modify/analyze a Call/Extensions during GetCall().

api/agent/agent.go

@@ -120,6 +120,8 @@ type agent struct {
 	shutonce            sync.Once
 	callEndCount        int64
 	disableAsyncDequeue bool
+
+	callOverrider CallOverrider
 }
 
 type AgentOption func(*agent) error
@@ -194,6 +196,17 @@ func WithoutAsyncDequeue() AgentOption {
 	}
 }
 
+// Agents can use this to register a CallOverrider to modify a Call and extensions
+func WithCallOverrider(fn CallOverrider) AgentOption {
+	return func(a *agent) error {
+		if a.callOverrider != nil {
+			return errors.New("lb-agent call overriders already exists")
+		}
+		a.callOverrider = fn
+		return nil
+	}
+}
+
 // Create a default docker driver from agent config
 func NewDockerDriver(cfg *AgentConfig) *docker.DockerDriver {
 	return docker.NewDocker(drivers.Config{