MM-26056 Add plugin ability to execute slash command. (#14898)

* Add plugin ability to execute slash command.

* A test

* Add site url to command args for plugin API

* Remove unessiary complexity.

plugin/plugintest/api.go

@@ -483,6 +483,29 @@ func (_m *API) EnablePlugin(id string) *model.AppError {
 	return r0
 }
 
+// ExecuteSlashCommand provides a mock function with given fields: commandArgs
+func (_m *API) ExecuteSlashCommand(commandArgs *model.CommandArgs) (*model.CommandResponse, error) {
+	ret := _m.Called(commandArgs)
+
+	var r0 *model.CommandResponse
+	if rf, ok := ret.Get(0).(func(*model.CommandArgs) *model.CommandResponse); ok {
+		r0 = rf(commandArgs)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(*model.CommandResponse)
+		}
+	}
+
+	var r1 error
+	if rf, ok := ret.Get(1).(func(*model.CommandArgs) error); ok {
+		r1 = rf(commandArgs)
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
+}
+
 // GetBot provides a mock function with given fields: botUserId, includeDeleted
 func (_m *API) GetBot(botUserId string, includeDeleted bool) (*model.Bot, *model.AppError) {
 	ret := _m.Called(botUserId, includeDeleted)