Actor principals for AWS and AWS-like services.
Principals come in two "flavors": actor principals and policy principals. Policy principals are used in Aspen documents and have a source ("AWS", "CanonicalUser", "Federated", or "Service") and an associated value which may contain wildcards. These are implemented in the scratchstack-aspen crate.
On the service implementation side, actor principals (represented by the [Principal] enum here) are exact, without
wildcards. Beyond the core details, there are additional details attached to a principal actor that can be referenced in
policy variables.
For example, IAM users have a
universally unique ID.
If the /Sales/Bob user is deleted and re-created, these two users will have the same ARN but different unique IDs
that can be referenced via the aws:userid condition key. These details are carried in [SessionData] structures
apart from the [Principal] itself.