add UpdateUserRoles to plugin api (#26615)

* ProfileImageBytes for EnsureBotOptions

* leverage plugintest.NewAPI

* fix linting

* add UpdateUserRoles to plugin api

server/channels/app/plugin_api_tests/test_update_user_roles_plugin/main.go

@@ -0,0 +1,52 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package main
+
+import (
+	"fmt"
+
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/plugin"
+	"github.com/mattermost/mattermost/server/v8/channels/app/plugin_api_tests"
+)
+
+type MyPlugin struct {
+	plugin.MattermostPlugin
+	configuration plugin_api_tests.BasicConfig
+}
+
+func (p *MyPlugin) OnConfigurationChange() error {
+	if err := p.API.LoadPluginConfiguration(&p.configuration); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (p *MyPlugin) MessageWillBePosted(_ *plugin.Context, _ *model.Post) (*model.Post, string) {
+	if p.API.HasPermissionTo(p.configuration.BasicUserID, model.PermissionManageSystem) {
+		return nil, "basic user should not yet be a system admin"
+	}
+
+	if _, appErr := p.API.UpdateUserRoles(p.configuration.BasicUserID, model.SystemAdminRoleId+" "+model.SystemUserRoleId); appErr != nil {
+		return nil, fmt.Sprintf("failed to update user roles: %s", appErr)
+	}
+
+	if !p.API.HasPermissionTo(p.configuration.BasicUserID, model.PermissionManageSystem) {
+		return nil, "basic user should be a system admin"
+	}
+
+	if _, appErr := p.API.UpdateUserRoles(p.configuration.BasicUserID, model.SystemUserRoleId); appErr != nil {
+		return nil, fmt.Sprintf("failed to update user roles: %s", appErr)
+	}
+
+	if p.API.HasPermissionTo(p.configuration.BasicUserID, model.PermissionManageSystem) {
+		return nil, "basic user should no longer be a system admin"
+	}
+
+	return nil, "OK"
+}
+
+func main() {
+	plugin.ClientMain(&MyPlugin{})
+}