The purpose of this class is to provide a general client for all APIs
available at https://download.moodle.org/api/ (e.g. available updates,
plugin info, plugins list etc). Currently, fetching data from this API
is done separately at several places. This leads to code duplication and
harder maintenance (I know it well).
Additionally, the existing client was implemented as
tool_installaddon_pluginfo_client in the admin/tool/installaddon/ scope.
I will soon need to use the same functionality in the
core_plugin_manager and it would hurt my karma if the core was depending
on a class provided by a admin tool plugin (even if it is standard one).
So, there is new \core\update\api client implementing the version 1.3 of
the pluginfo API. There is a TODO note left for remaining services.
