Add anonymous user flag

[kiootic]

ref #1370

• Add anonymous user flag in user model
• Add anonymous user flag header in session resolver
• Update SDK user model

[kiootic]

Since user API model creation is tightly coupled within Auth API handlers. I'll postpone the update of user model until we've decided whether to drop Auth API in #1434 .

[kiootic]

Workarounds at the moment:

• Detect signup of anonymous users: use after_user_create hook and check if any of the identities is of anonymous type.
• Detect promotion of anonymous user to normal user: use after_identity_delete hook and check if any of the deleted identities is of anonymous type.