feat: add support for custom fs provider

[CompuIves]

We're looking to add our own providers for user and cache fs so they can be persisted between sessions. With this change, we don't break the existing API but we can allow for adding custom filesystems for these providers.

[CompuIves]

I'm also exposing IndexedDBFileSystemProvider, which I want to use for the user store. Is it okay if that's part of this PR?

[CompuIves]

While I'm testing the code, I can't seem to get the customProviders inside the constructor. I was under the assumption that if it's passed to the staticArguments array, it would be passed as the second argument in the constructor, but it's empty for me. Have you seen something like this before?

[CompuIves]

Ah, found that static arguments need to be passed as first argument, and then the decorators. I'll try that now.

[CompuIves]

That didn't work either. It looks like another place calls createInstance on it (in my case, the simple extension service) and that causes the static argument to be lost. I have another way we can solve this, but it's less nice. I'll commit that as part of the first commit now.

[CompuIves]

Ah, I found the issue. The getServiceOverride is called multiple times, also by other services. Because of this, I've decided to create a specific function to override the different filesystem providers. That ensures that we don't accidentally reset them.