Implement a validation webhook

In case some ingress have a syntax error in the snippet configuration,
the freshly generated configuration will not be reloaded to prevent tearing down existing rules.
Although, once inserted, this configuration is preventing from any other valid configuration to be inserted as it remains in the ingresses of the cluster.
To solve this problem, implement an optional validation webhook that simulates the addition of the ingress to be added together with the rest of ingresses.
In case the generated configuration is not validated by nginx, deny the insertion of the ingress.

In case certificates are mounted using kubernetes secrets, when those
changes, keys are automatically updated in the container volume, and the
controller reloads it using the filewatcher.

Related changes:

- Update vendors
- Extract useful functions to check configuration with an additional ingress
- Update documentation for validating webhook
- Add validating webhook examples
- Add a metric for each syntax check success and errors
- Add more certificate generation examples

cmd/nginx/flags.go

@@ -159,6 +159,14 @@ Feature backed by OpenResty Lua libraries. Requires that OCSP stapling is not en
 
 		disableCatchAll = flags.Bool("disable-catch-all", false,
 			`Disable support for catch-all Ingresses`)
+
+		validationWebhook = flags.String("validating-webhook", "",
+			`The address to start an admission controller on to validate incoming ingresses.
+Takes the form "<host>:port". If not provided, no admission controller is started.`)
+		validationWebhookCert = flags.String("validating-webhook-certificate", "",
+			`The path of the validating webhook certificate PEM.`)
+		validationWebhookKey = flags.String("validating-webhook-key", "",
+			`The path of the validating webhook key PEM.`)
 	)
 
 	flags.MarkDeprecated("status-port", `The status port is a unix socket now.`)
@@ -255,7 +263,10 @@ Feature backed by OpenResty Lua libraries. Requires that OCSP stapling is not en
 			HTTPS:    *httpsPort,
 			SSLProxy: *sslProxyPort,
 		},
-		DisableCatchAll: *disableCatchAll,
+		DisableCatchAll:           *disableCatchAll,
+		ValidationWebhook:         *validationWebhook,
+		ValidationWebhookCertPath: *validationWebhookCert,
+		ValidationWebhookKeyPath:  *validationWebhookKey,
 	}
 
 	return false, config, nil