Framework for migration of plugin data into core tables

[cedric-anne]

Checklist before requesting a review

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• This change requires a documentation update.

Description

This is not yet finished.

Remaining tasks:

• store more details in the MigrationResult (detailled count of imported/updated objects);
• handle an optional progress indicator to output the results in real-time;
• handle a generic result output in the console;
• add many tests.

The purpose of this "framework" is to ease the developement and the testing of the migration of plugins data into core tables. It will be used to handle the migration of the formcreator and the genericobject data in GLPI 11.0.