Use a Server Mode to add datetime_t Attributes Across Event Classes

[pagbabian-splunk]

For the new datetime_t data type, Proposal 11 had a profile that would overlay a companion attribute everywhere there is a timestamp_t data type. Rather than use an actual profile, the schema server can have a switch or mode where this can be done globally (i.e. wherever there is a timestamp_t attribute in any event class). How that is detected via the API or schema is yet to be determined but I like the idea. Otherwise it is the same as Proposal 11. The original timestamp_t field must still be populated.