feat: added before load event for objects, documents and assets

[zoidbergx]

Changes in this pull request

Additional info

added pre load events for objects, documents and assets

WHAT

we need to check some stuff before a "entity" (object, document, asset) is loaded and need to be able to return nothing respective throw an exception which is the same as an entity not exists.

As there are no other possiblities to hook in into the DB-queries this seems to be the cleanest solution.

[github-actions]

Review Checklist

• Target branch (11.0 for bug fixes, others 11.x)
• Tests (if it's testable code, there should be a test for it - get help)
• Docs (every functionality needs to be documented, see here)
• Migration incl. install.sql (e.g. if the database schema changes, ...)
• Upgrade notes (deprecations, important information, migration hints, ...)
• Label
• Milestone