MM-10702 Moving plugins to use hashicorp go-plugin. (#8978)

* Moving plugins to use hashicorp go-plugin.

* Tweaks from feedback.

plugin/example_hello_world_test.go

@@ -1,20 +0,0 @@
-package plugin_test
-
-import (
-	"fmt"
-	"net/http"
-
-	"github.com/mattermost/mattermost-server/plugin/rpcplugin"
-)
-
-type HelloWorldPlugin struct{}
-
-func (p *HelloWorldPlugin) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	fmt.Fprintf(w, "Hello, world!")
-}
-
-// This example demonstrates a plugin that handles HTTP requests which respond by greeting the
-// world.
-func Example_helloWorld() {
-	rpcplugin.Main(&HelloWorldPlugin{})
-}