Support changing object storage providers

[Gabe-LSN]

Summary

Misskey supports object storage, but if you change providers you must change the location of each file in the database, just as if you wanted to move locally stored files to object storage.

In my opinion, it would be best for misskey to pull the image location from object storage if it's configured, potentially migrating locally stored images if they exist. Otherwise, merely being able to update the location of all media from one object storage provider to another would allow users to migrate their images from different hosts if necessary.

[Sayamame-beans]

triage: 連合面が厄介そうですかね?