ComplexSubject should have a format field

[FragLegs]

The ComplexSubject identifier type contains the following fields:

• user
• device
• session
• application
• tenant
• org_unit
• group

All SimpleSubject types have a format field that identifies the type and then some number of other fields specific to the type. By not providing a format field in ComplexSubject, it has to be parsed differently. And, importantly, a Receiver has no way to request that a Stream Configuration use ComplexSubjects as the format value (see section 7.1.1 Stream Configuration).

I propose that we add a format field for ComplexSubjects and the value be complex.