oidc: use shared library from LXD when ready

## Issue description

Currently in site manager, we have an internal oidc package which is based on the [oidc package](https://github.com/canonical/lxd/blob/3b272e24010e55e46388fa8d312f939cc7421405/lxd/auth/oidc/oidc.go#L1) from LXD. There are some plans to update the LXD package so that it becomes a reusable library. Once it is ready, we should import it as a dependency and remove the internal oidc package in site manager.