MDL-40457 Question Bank: Allow plugins to add columns

Allows plugins to add columns to the question bank view by extending question_bank_column_base.
The plugin should return an array of its columns from *_question_bank_column_types().
Columns to display are set in $CFG->questionbankcolumns.
Merge branch 'MDL-40457-question_bank_plugins_columns' of github.com:MorrisR2/moodle into MDL-40457-question_bank_plugins_columns