IBX-2823: Added loadRelation method

[ciastektk]

Question	Answer
JIRA issue	IBX-2823
Type	improvement
Target Ibexa version	v4.1
BC breaks	no

List of changes:

• Changed method signature Ibexa\Contracts\Core\Persistence\Content\Handler::removeRelation - added optional third argument $destinationContentId which will be used to invalidate cache tag
• Added method loadRelation to Ibexa\Contracts\Core\Persistence\Content\Handler to load relation by id
• Added method loadRelation to Ibexa\Core\Persistence\Legacy\Content\Gateway to get relation data from database
• Added cache usage in Ibexa\Core\Persistence\Cache\ContentHandler::countReverseRelations for better performance
• Added new cache identifiers content_reverse_relations_count - crrc and relation - re
• Passed $destinationContentId where removeRelation method is called

Checklist:

• Provided PR description.
• Tested the solution manually.
• Provided automated test coverage.
• Checked that target branch is set correctly (main for features, the oldest supported for bugs).
• Ran PHP CS Fixer for new PHP code (use $ composer fix-cs).
• Asked for a review (ping @ibexa/engineering).

[tomaszszopinski]

QA approved on IbexaDXP 4.1 commerce.