Allow Requests to be Mirrored to different backends

Add a feature which allows traffic to be mirrored to
additional backends. This is useful for testing how
requests will behave on different "test" backends.

See https://nginx.org/en/docs/http/ngx_http_mirror_module.html

internal/ingress/annotations/mirror/main.go

@@ -0,0 +1,58 @@
+/*
+Copyright 2019 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package mirror
+
+import (
+	networking "k8s.io/api/networking/v1beta1"
+
+	"k8s.io/ingress-nginx/internal/ingress/annotations/parser"
+	"k8s.io/ingress-nginx/internal/ingress/resolver"
+)
+
+// Config returns the mirror to use in a given location
+type Config struct {
+	URI         string `json:"uri"`
+	RequestBody string `json:"requestBody"`
+}
+
+type mirror struct {
+	r resolver.Resolver
+}
+
+// NewParser creates a new mirror configuration annotation parser
+func NewParser(r resolver.Resolver) parser.IngressAnnotation {
+	return mirror{r}
+}
+
+// ParseAnnotations parses the annotations contained in the ingress
+// rule used to configure mirror
+func (a mirror) Parse(ing *networking.Ingress) (interface{}, error) {
+	config := &Config{}
+	var err error
+
+	config.URI, err = parser.GetStringAnnotation("mirror-uri", ing)
+	if err != nil {
+		config.URI = ""
+	}
+
+	config.RequestBody, err = parser.GetStringAnnotation("mirror-request-body", ing)
+	if err != nil || config.RequestBody != "off" {
+		config.RequestBody = "on"
+	}
+
+	return config, nil
+}