Adding interplugin communication. (#12829)

* Adding interplugin communication.

* Naming changes and moving ResponseTransfer to own file.

* Fix.

* Tests and moving to buffering bytes.

* Switching API to passing plugin ID through path rather than a header.

* Review feedback.

plugin/context.go

@@ -12,4 +12,5 @@ type Context struct {
 	IpAddress      string
 	AcceptLanguage string
 	UserAgent      string
+	SourcePluginId string
 }