oidc: Improve approach for handling OIDC login flow across different cluster members

# Issue description

The OIDC login will fail in a LXD cluster if different members handle different stages of the flow. This is likely to occur if load balancing is deployed in front of the cluster.

This is an open [issue](https://github.com/canonical/lxd/issues/13644) in LXD. Currently in site manager, we have implemented a work around by setting the member address in the `state` query param as part of the idp callback url. This allows the `/oidc/callback` endpoint to identify which member initiated the login flow and therefore forward the request to that member for token exchange. However, there may be a better solution implemented in LXD. We should align the approach in site manager with that in LXD once the issue is closed.