File provider does not use NewKoanfFileSubKey

[isodude]

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.
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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: c1adb0d

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