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/handler.go

@@ -0,0 +1,24 @@
+package header
+
+import (
+	"context"
+
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/miekg/dns"
+)
+
+// Header modifies dns.MsgHdr in the responses
+type Header struct {
+	Rules []Rule
+	Next  plugin.Handler
+}
+
+// ServeDNS implements the plugin.Handler interface.
+func (h Header) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	wr := ResponseHeaderWriter{ResponseWriter: w, Rules: h.Rules}
+	return plugin.NextOrFailure(h.Name(), h.Next, ctx, &wr, r)
+}
+
+// Name implements the plugin.Handler interface.
+func (h Header) Name() string { return "header" }