Slack Guest User support for Unfurling

With our recent updates to unfurling, we require that users authenticate themselves with Atlassian, so that we know who they are and can guarantee they have access to the issue they are trying to unfurl. 

Unfortunately, Slack Guest Users are unable to authenticate themselves because we require that the user authenticates with Slack, something guest users in Slack are incapable of doing. 

It is possible for us to just authenticate the Guest User on the Atlassian side, although they won't be able to take any actions (via the "..." menu) or use slash commands to create Jira issues, as we won't be able to authenticate them on the Slack side.

This feature request is to support guest users to be able to unfurl via a secondary authentication method that does not exist and needs to be built