Add GET /user_management/users/:id/feature-flags support (#1364)

## Description

- Add GET /user_management/users/:id/feature-flags support with a
`listFeatureFlags` method on the users module

## Documentation

Does this require changes to the WorkOS Docs? E.g. the [API
Reference](https://workos.com/docs/reference) or code snippets need
updates.

```
[x] Yes
```

If yes, link a related docs PR and add a docs maintainer as a reviewer.
Their approval is required.