Fix GitLab plugins not working without hooks configured

gitlab-foss!31741 (merged), shipped in GitLab 12.2, reduced Gitaly RPCs by generating commit data only if hooks or services were actually active. However, GitLab plugins (https://docs.gitlab.com/ee/administration/plugins.html), are executable programs that get launched inside SystemHooksService#execute_hooks regardless of whether any hooks are actually active. As a result of this change, users have to configure a Webhook for plugins to fire.

We restore the previous behavior by checking whether plugins are available and executing SystemHooksService#execute_hooks if any plugins are available.

Closes #194131 (closed)