[Keycloak] Support relative paths for health checks

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

When hosting keycloak behind a reverse proxy (e.g. YARP, sub path is `/auth`), the health checks don't work.

I think the reason is this line:

```
.WithHttpHealthCheck(endpointName: ManagementEndpointName, path: "/health/ready")
```

### Describe the solution you'd like

It would be good if the keycloak extension would allow specification of a relative path.

This workaround works, but the default health check still fails:

```
.WithHttpHealthCheck(endpointName: "management", path: "/auth/health/ready");
```

### Additional context

_No response_