File provider does not use NewKoanfFileSubKey

Preflight checklist

• I could not find a solution in the existing issues, docs, nor discussions.
• I agree to follow this project's Code of Conduct.
• I have read and am following this repository's Contribution Guidelines.
• This issue affects my Ory Cloud project.
• I have joined the Ory Community Slack.
• I am signed up to the Ory Security Patch Newsletter.

Describe the bug

I assume that in https://github.com/ory/x/blob/master/configx/provider.go NewKoanfFileSubKey should used such that it's possible to specify which node a config file should apply to.

Right now it seems to be dead code. If not maybe document how to use it?

Feature: https://github.com/ory/x/commit/c1adb0d66d226c8ea1e2f6fe1ffa3832c639b780

Reproducing the bug

Read the code :)

Relevant log output

No response

Relevant configuration

No response

Version

Keto 0.9.0

On which operating system are you observing this issue?

Linux

In which environment are you deploying?

Binary

Additional Context

No response

Start in configx/provider.go and compare the current file-provider behavior with the NewKoanfFileSubKey feature introduced in commit c1adb0d66d226c8ea1e2f6fe1ffa3832c639b780. Determine whether the subkey option is intended to affect configuration loading or only needs documentation; done means resolving that ambiguity with the corresponding code or documentation change.