Split weapon attachments into weapon and class-based categories

Then you can allow players to universally unlock all weapon attachments across classes, but to get the class-specific attachments they have to put time in as those classes

e.g. M4 is available to both Assault & Medic
Assault could get extended mag
Medic gets underbarrel grenade launcher
———
MP5 is available to Engi & Medic
Engi gets 4x scope
Medic gets suppressor

Even if only 1 or 2 attachments are class-locked, it'll still require a bit of a grind for the weapons without feeling like a complete restart