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/setup.go

@@ -0,0 +1,50 @@
+package header
+
+import (
+	"fmt"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/caddy"
+)
+
+func init() { plugin.Register("header", setup) }
+
+func setup(c *caddy.Controller) error {
+	rules, err := parse(c)
+	if err != nil {
+		return plugin.Error("header", err)
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		return Header{
+			Rules: rules,
+			Next:  next,
+		}
+	})
+
+	return nil
+}
+
+func parse(c *caddy.Controller) ([]Rule, error) {
+	for c.Next() {
+		var all []Rule
+		for c.NextBlock() {
+			v := c.Val()
+			args := c.RemainingArgs()
+			// set up rules
+			rules, err := newRules(v, args)
+			if err != nil {
+				return nil, fmt.Errorf("seting up rule: %w", err)
+			}
+			all = append(all, rules...)
+		}
+
+		// return combined rules
+		if len(all) > 0 {
+			return all, nil
+		}
+	}
+	return nil, c.ArgErr()
+
+}