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/README.md

@@ -0,0 +1,52 @@
+# header
+
+## Name
+
+*header* - modifies the header for all the responses.
+
+## Description
+
+It ensures that the flags are in the desired state for all the responses. The modifications are made transparently for
+the client.
+
+## Syntax
+
+~~~
+header {
+    ACTION FLAGS...
+    ACTION FLAGS...
+}
+~~~
+
+* **ACTION** defines the state for dns flags. Actions are evaluated in the order they are defined so last one has the
+  most precedence. Allowed values are:
+    * `set`
+    * `clear`
+* **FLAGS** are the dns flags that will be modified. Current supported flags include:
+    * `aa` - Authoritative
+    * `ra` - RecursionAvailable
+    * `rd` - RecursionDesired
+
+## Examples
+
+Make sure recursive available `ra` flag is set in all the responses:
+
+~~~ corefile
+. {
+    header {
+        set ra
+    }
+}
+~~~
+
+Make sure recursive available `ra` and authoritative `aa` flags are set and recursive desired is cleared in all the
+responses:
+
+~~~ corefile
+. {
+    header {
+        set ra aa 
+        clear rd
+    }
+}
+~~~