fix: make apm config a pointer (#2995)

This commit makes the APM configuration a pointer so that we can
properly nil check it in the handler.

Author: betterengineering

api/datadoghq/v1alpha1/datadoginstrumentation_types.go

@@ -30,7 +30,7 @@ type DatadogInstrumentationConfig struct {
 
 	// APM configures the APM product through Single Step Instrumentation for the target workload.
 	// +optional
-	APM DatadogInstrumentationAPMConfig `json:"apm,omitempty"`
+	APM *DatadogInstrumentationAPMConfig `json:"apm,omitempty"`
 }
 
 // DatadogInstrumentationAPMConfig defines workload-scoped APM configuration.