Require defining a begin config for microbatch incremental models

[QMalcolm]

We need to know what the "beginning of time" is for a microbatch model, to know when to start for these two cases:

• The first time this model is run
• If you run in “full-refresh-mode”

This is distinct from the CLI parameter --event-time-start which can also be used when running in "incremental" mode.