[Id-event] Common claims in subject identifiers

[Atul Tulshibagwale <[e-mail]>]

Hi all,
The proposed subject identifiers specification today requires that each
claim within a subject identifier be specified by the individual subject
identifier type. However, there may be some common claims that could be
defined across all subject identifier types. These are captured within the
"common claims" section of my proposal. All such common claims may be
optional unless a specific subject identifier type requires any of them
(there is no example of that today). These claims include:

   - *SPAG Identifier*: Identifies the "Subject Principal Administrative
   Grouping" or SPAG that the subject identifier belongs to
   - *Subject Category*: identifies the category that the subject
   identifier refers to (whether user, device or session)

To avoid having to define these claims within each subject identifier type,
I have extracted them into a separate section called "common claims" in my
proposal <https://github.com/richanna/secevent/pull/1>.

Thanks,
Atul