plugin/view: Advanced routing interface and new 'view' plugin (#5538)

* introduce new interface "dnsserver.Viewer", that allows a plugin implementing it to decide if a query should be routed into its server block.
* add new plugin "view", that uses the new interface to enable a user to define expression based conditions that must be met for a query to be routed to its server block.

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/view/metadata.go

@@ -0,0 +1,16 @@
+package view
+
+import (
+	"context"
+
+	"github.com/coredns/coredns/plugin/metadata"
+	"github.com/coredns/coredns/request"
+)
+
+// Metadata implements the metadata.Provider interface.
+func (v *View) Metadata(ctx context.Context, state request.Request) context.Context {
+	metadata.SetValueFunc(ctx, "view/name", func() string {
+		return v.viewName
+	})
+	return ctx
+}