MDL-61905 workshop: Implement the privacy API in the workshop core

MDL-61905 workshop: Implement the privacy API in the workshop core

Workshop module stores personal data in its tables, via user
preference and via core_files and core_plagiarism subsystems.

When exporting the data, we export not only data created by users
themselves (such as their submissions and provided peer-assessments) but
also all relevant data that can (or must) be used to interpret created
content and evaluate the user's performance and skills.

On the other hand, when deleting data at user's request, we delete only
those data that do not affect other users' performance evaluation. The
reasoning is that one's right for privacy does not overweight someone
else's right for fair assessment. For that reason, we can't fully delete
whole provided peer-assessments, for example. Because they are used in
cross-comparison and grading evaluation of all other peers who assessed
the same submission. So instead, we replace provided texts but still
keep the original record.

Workshop defines the interface for its grading strategy subplugins to
allow them attach personal data under their control to the exported
structures.