Expose finer grained offer veto stats

TaskVars expose scheduling offer veto stats based on VetoGroup (scheduling_veto_*). The STATIC veto group consists of four different veto types (INSUFFICIENT_RESOURCES, CONSTRAINT_MISMATCH, MAINTENANCE, and DEDICATED_CONSTRAINT_MISMATCH). As is, these metrics are not very useful.

Consider exposing finer grained veto stats based on VetoType.