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

* Moving plugins to use hashicorp go-plugin.

* Tweaks from feedback.

plugin/client.go

@@ -0,0 +1,51 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package plugin
+
+import (
+	"github.com/hashicorp/go-plugin"
+)
+
+// Starts the serving of a Mattermost plugin over rpc or gRPC
+// Call this when your plugin is ready to start
+func ClientMain(pluginImplementation interface{}) {
+	if impl, ok := pluginImplementation.(interface {
+		SetAPI(api API)
+		SetSelfRef(ref interface{})
+	}); !ok {
+		panic("Plugin implementation given must embed plugin.MattermostPlugin")
+	} else {
+		impl.SetAPI(nil)
+		impl.SetSelfRef(pluginImplementation)
+	}
+
+	pluginMap := map[string]plugin.Plugin{
+		"hooks": &HooksPlugin{hooks: pluginImplementation},
+	}
+
+	plugin.Serve(&plugin.ServeConfig{
+		HandshakeConfig: Handshake,
+		Plugins:         pluginMap,
+	})
+}
+
+type MattermostPlugin struct {
+	API     API
+	selfRef interface{} // This is so we can unmarshal into our parent
+}
+
+func (p *MattermostPlugin) SetAPI(api API) {
+	p.API = api
+}
+
+func (p *MattermostPlugin) SetSelfRef(ref interface{}) {
+	p.selfRef = ref
+}
+
+func (p *MattermostPlugin) OnConfigurationChange() error {
+	if p.selfRef != nil {
+		return p.API.LoadPluginConfiguration(p.selfRef)
+	}
+	return nil
+}