Implement a new callback to extend gradebook plugininfo

Problem statement:
We would like to add tabs in the gradebook for custom functionality that does not necessarily fall into the current gradebook buckets of "Reports, Settings, Scale, Outcome, Letter, Import, Export" plugins.

Proposed solution:
We propose to implement a hook in the grade_get_plugin_info function in grade/lib.php so that we can manipulate the $plugin_info variable to either add or remove tabs from the gradebook as necessary.