Plugin framework: add ability to install other plugins to the… (#12232)

* add ability to upload other plugins to the plugin API

* generated client rpc glue code

* fix UploadPlugin API signature

* generated plugin mocks

* added upload plugin test

* removed unused comment

* using single line to call InstallPlugin with file Reader

* fix minimum server version

* added successful plugin upload test

* renamed UploadPlugin to InstallPlugin

plugin/api.go

@@ -4,6 +4,8 @@
 package plugin
 
 import (
+	"io"
+
 	plugin "github.com/hashicorp/go-plugin"
 	"github.com/mattermost/mattermost-server/model"
 )
@@ -557,6 +559,12 @@ type API interface {
 	// Minimum server version: 5.6
 	GetPluginStatus(id string) (*model.PluginStatus, *model.AppError)
 
+	// InstallPlugin will upload another plugin with tar.gz file.
+	// Previous version will be replaced on replace true.
+	//
+	// Minimum server version: 5.18
+	InstallPlugin(file io.Reader, replace bool) (*model.Manifest, *model.AppError)
+
 	// KV Store Section
 
 	// KVSet stores a key-value pair, unique per plugin.