Sandboxing for MFS, Keys, IPNS

With the addition of #10187, the next natural step is to allow sandboxing for the different APIs according to the given Authorization keys. At the moment, the following are identified APIs that should be sandboxed:

- MFS: each application will have their own MFS that does not collide with others.
- Keys: each application can only see and access keys that they have created.
- IPNS: each application can only interact with keys that they own (related to above).

This will be useful for applications built on top of the new authenticated API.

Easiest solution is to prepend some path to MFS paths and key names.

cc @lidel if you want to add more context

cc https://github.com/brave/brave-browser/issues/34000 as Brave would like to use this for sandboxing access per blessed extension like https://webrecorder.net/