Add ability to check environment before installing addons

Occasionally an addon will require the installation of an additional PHP extension or have version requirements. Currently, we have the maintainer manually marking PHP version in Moodle Plugins; however, I would like to see an additional option where a maintainer could provide an environment.xml file that lists any additional requirements beyond that particular version of Moodle. For example, a plugin may take advantage of a later version of PHP than core does. This would protect sites from installing code for which the environment does not support it.