Remove Plugin hard dependency from PermissionAttachment

[dktapps]

Problem description

This dependency makes it inconvenient to create PermissionAttachments within the core.

This would be useful for temporary permission overrides internally, e.g. gameplay permission overrides for #5544 that disappear when the player's gamemode is changed.

Proposed solution

Make the Plugin parameter optional, or get rid of it entirely - depends whether it's actually useful for anything or not.

Alternative solutions that don't require API changes