Directory sync role mapping

[jamilbk]

When users are synced from an identity provider to Firezone, a final step is required to promote any admin users that need access to the admin portal. This is because synced users are always "regular" users by default.

To save this step, we can allow for incoming role to be mapped to the Firezone admin role upon syncing. The purpose of this issue is to provide a staging ground to gather feedback and further discussion on the feature.

Refs #5906