Add "Enterprise HST" dependencies to enterprise plugin

CMS 12.3 introduces the com.onehippo.cms7:hippo-addon-enterprise-site-toolkit-dependencies-server module, to be added as a dependency to your project's site/pom.xml.

We should improve Essentials' "Enterprise Project" plugin to also install this dependency when converting a community project to an enterprise project.

This issue requires a trivial effort: add a <mavenDependency> instruction to the plugin's instructions file, and you're done. We may also want to update the documentation at https://cms.onehippo.com/?1&path=/content/documents/12/library/concepts/spa-plus/installation when this is in place.