Fix assets dropdowns visibility from self service

[AdrienClairembault]

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.

Description

On GLPI 10, ajax request to /getDropdownValue would return their own assets for self service users.

See on formcreator, logged in as 'post-only':

This seems to have changed in GLPI 11, possibly by accident while introducing the generic "AssignableItem" trait.
This PR rectify this behavior by:

• Creating two dedicated getAssignableVisiblityCriteriaForCentral and getAssignableVisiblityCriteriaForHelpdesk methods.
• The original getAssignableVisiblityCriteria method will call the correct submethod depending on the current interface
• The getAssignableVisiblityCriteriaForHelpdesk method will return a criteria that will allow the user to see his own items, without right checks as such rights are not available for helpdesk profiles
• A getOwnAssetsCriteria method was introduced to share logic without duplication between the two new methods.

References

Fix #21509, #21566 and #21502.

[AdrienClairembault]

Back to draft, I think I need to take this setting into account too: