plugin/header: Introduce header plugin (#4752)

* Add header plugin

Signed-off-by: MQasimSarfraz <syed.qasim.sarfraz@gmail.com>

* fix import format

* improve README.md

* Add codeowners for header plugin

plugin/header/header_test.go

@@ -0,0 +1,84 @@
+package header
+
+import (
+	"context"
+	"testing"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+func TestHeader(t *testing.T) {
+	wr := dnstest.NewRecorder(&test.ResponseWriter{})
+	next := plugin.HandlerFunc(func(ctx context.Context, writer dns.ResponseWriter, msg *dns.Msg) (int, error) {
+		writer.WriteMsg(msg)
+		return dns.RcodeSuccess, nil
+	})
+
+	tests := []struct {
+		handler  plugin.Handler
+		got      func(msg *dns.Msg) bool
+		expected bool
+	}{
+		{
+			handler: Header{
+				Rules: []Rule{{Flag: recursionAvailable, State: true}},
+				Next:  next,
+			},
+			got: func(msg *dns.Msg) bool {
+				return msg.RecursionAvailable
+			},
+			expected: true,
+		},
+		{
+			handler: Header{
+				Rules: []Rule{{Flag: recursionAvailable, State: true}},
+				Next:  next,
+			},
+			got: func(msg *dns.Msg) bool {
+				return msg.RecursionAvailable
+			},
+			expected: true,
+		},
+		{
+			handler: Header{
+				Rules: []Rule{{Flag: recursionDesired, State: true}},
+				Next:  next,
+			},
+			got: func(msg *dns.Msg) bool {
+				return msg.RecursionDesired
+			},
+			expected: true,
+		},
+		{
+			handler: Header{
+				Rules: []Rule{{Flag: authoritative, State: true}},
+				Next:  next,
+			},
+			got: func(msg *dns.Msg) bool {
+				return msg.Authoritative
+			},
+			expected: true,
+		},
+	}
+
+	for i, test := range tests {
+		m := new(dns.Msg)
+
+		_, err := test.handler.ServeDNS(context.TODO(), wr, m)
+		if err != nil {
+			t.Errorf("Test %d: Expected no error, but got %s", i, err)
+			continue
+		}
+
+		if test.got(m) != test.expected {
+			t.Errorf("Test %d: Expected flag state=%t, but got %t", i, test.expected, test.got(m))
+			continue
+		}
+
+	}
+
+}