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/plugintest/api.go

@@ -6,9 +6,11 @@ package plugintest
 
 import (
 	io "io"
+	http "net/http"
+
+	mock "github.com/stretchr/testify/mock"
 
 	model "github.com/mattermost/mattermost-server/model"
-	mock "github.com/stretchr/testify/mock"
 )
 
 // API is an autogenerated mock type for the API type
@@ -2334,6 +2336,22 @@ func (_m *API) PermanentDeleteBot(botUserId string) *model.AppError {
 	return r0
 }
 
+// PluginHTTP provides a mock function with given fields: request
+func (_m *API) PluginHTTP(request *http.Request) *http.Response {
+	ret := _m.Called(request)
+
+	var r0 *http.Response
+	if rf, ok := ret.Get(0).(func(*http.Request) *http.Response); ok {
+		r0 = rf(request)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(*http.Response)
+		}
+	}
+
+	return r0
+}
+
 // PublishWebSocketEvent provides a mock function with given fields: event, payload, broadcast
 func (_m *API) PublishWebSocketEvent(event string, payload map[string]interface{}, broadcast *model.WebsocketBroadcast) {
 	_m.Called(event, payload, broadcast)