Separation of concerns for the Attachments class

[Menrath]

I propose splitting this class up in two classes.

Class Attachments: Reponsibly for parsing and transmogrifying ActivityPub objects.
Class Remote_Cache: Responsible for the storing and retrieving logic.

Why: I currently migrate the Event Bridge to the new ap_actor follows and also want to use this cache system for storing remote media which is used by the event plugins post types. But I need some additional features to make this possible, and at this point it really seemed that this class is doing too much and I got quite distracted from how to solve that.