Troubleshooting help - "cookies/headers too large with external identity provider"

See https://github.com/orgs/DuendeSoftware/discussions/23#discussioncomment-12242002

Elaborate on claims mapping, role claims potentially taking a lot of space, and go in a bit more detail at potential ways to go about this (including targeted code snippets e.g. on cleaning up the principal in the `OnTicketReceived` event.