add plugin http handler (#7289)

plugin/hooks.go

@@ -1,5 +1,9 @@
 package plugin
 
+import (
+	"net/http"
+)
+
 type Hooks interface {
 	// OnActivate is invoked when the plugin is activated.
 	OnActivate(API) error
@@ -7,4 +11,11 @@ type Hooks interface {
 	// OnDeactivate is invoked when the plugin is deactivated. This is the plugin's last chance to
 	// use the API, and the plugin will be terminated shortly after this invocation.
 	OnDeactivate() error
+
+	// ServeHTTP allows the plugin to implement the http.Handler interface. Requests destined for
+	// the /plugins/{id} path will be routed to the plugin.
+	//
+	// The Mattermost-User-Id header will be present if (and only if) the request is by an
+	// authenticated user.
+	ServeHTTP(http.ResponseWriter, *http.Request)
 }