Feature: Support symlinks to broker config

### Is there an existing request for this feature?

- [x] I have searched the existing issues and found none that matched mine

### Describe the feature

I'm not sure if this is also a bug.

When enabling a broker for authd, typically you would copy the broker config file as described in the documentation:

> `sudo cp /snap/authd-msentraid/current/conf/authd/msentraid.conf /etc/authd/brokers.d/`

But it might also be desirable to symlink the broker config file instead. This way you always have the most up-to-date broker config file sourced by authd after updating your broker snaps.

However, currently this doesn't work. If you symlink the config file, the only broker available is the "local" broker.

### Describe the ideal solution

_No response_

### Alternatives and current workarounds

_No response_

Start by reproducing the documented broker setup, comparing a copied config in /etc/authd/brokers.d/ with a symlink to the snap-provided config. Trace how authd discovers broker configurations and verify the result after a broker snap update. Done means the symlinked config is recognized and the broker is available alongside the local broker.

Written by the indexing model from the issue text.