Extend EventRecord type with user/group related data

We want to generate the following events:

• Unschedulable ask - not enough queue headroom
• Unschedulable ask - not enough user quota
• There is enough queue headroom for the ask
• There is enough user quota for the ask
• Unschedulable ask - predicate failure

Update SI constants with new event details.

Also, we need to emit UG-related events. So si.EventRecord needs to contain "user" and "group" fields or possibly a new type.