Add validation for feature-flags configmap to webhook

Prior to this commit, the validating admission webhook validated the logging,
defaults, and leader election configmaps, but not the feature flags configmap.
This meant that invalid edits to the feature-flags configmap would be silently
accepted, but could later cause PipelineRuns and TaskRuns to fail.
This commit validates the feature-flags configmap in the validation webhook.

Note: we currently have a cyclic dependency between the validating admission webhook
and the configmaps used by pipelines. This commit does not address this problem,
which should be handled separately (tektoncd#4542).

cmd/webhook/main.go

@@ -141,9 +141,10 @@ func newConfigValidationController(name string) func(context.Context, configmap.
 
 			// The configmaps to validate.
 			configmap.Constructors{
-				logging.ConfigMapName():               logging.NewConfigFromConfigMap,
-				defaultconfig.GetDefaultsConfigName(): defaultconfig.NewDefaultsFromConfigMap,
-				pkgleaderelection.ConfigMapName():     pkgleaderelection.NewConfigFromConfigMap,
+				logging.ConfigMapName():                   logging.NewConfigFromConfigMap,
+				defaultconfig.GetDefaultsConfigName():     defaultconfig.NewDefaultsFromConfigMap,
+				pkgleaderelection.ConfigMapName():         pkgleaderelection.NewConfigFromConfigMap,
+				defaultconfig.GetFeatureFlagsConfigName(): defaultconfig.NewFeatureFlagsFromConfigMap,
 			},
 		)
 	}