Replace SpecialUsers enum with full Principal object

[DiamondJoseph]

Adding type hints to where get_current_principal is used, there are special cases to consider with SpecialUsers. If the SpecialUsers were instead Principal objects, this special handling could be removed.

i.e. replace

SpecialUsers(str, Enum):
    admin = "admin"
    public = "public"

# with

admin = Principal(roles=...)
public = Principal(roles=...)