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_test.go

@@ -0,0 +1,53 @@
+package header
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/coredns/caddy"
+)
+
+func TestSetupHeader(t *testing.T) {
+	tests := []struct {
+		input              string
+		shouldErr          bool
+		expectedErrContent string
+	}{
+		{`header {}`, true, "Wrong argument count or unexpected line ending after"},
+		{`header {
+					set
+}`, true, "invalid length for flags, at least one should be provided"},
+		{`header {
+					foo
+}`, true, "invalid length for flags, at least one should be provided"},
+		{`header {
+					foo bar
+}`, true, "unknown flag action=foo, should be set or clear"},
+		{`header {
+					set ra 
+}`, false, ""},
+		{`header {
+			set ra aa
+			clear rd
+}`, false, ""},
+	}
+
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.input)
+		err := setup(c)
+
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %d: Expected error but found none for input %s", i, test.input)
+		}
+
+		if err != nil {
+			if !test.shouldErr {
+				t.Errorf("Test %d: Expected no error but found one for input %s. Error was: %v", i, test.input, err)
+			}
+
+			if !strings.Contains(err.Error(), test.expectedErrContent) {
+				t.Errorf("Test %d: Expected error to contain: %v, found error: %v, input: %s", i, test.expectedErrContent, err, test.input)
+			}
+		}
+	}
+}