[SPARK-52218][SQL] Make current datetime functions evaluable again

[cloud-fan]

What changes were proposed in this pull request?

Current date-time functions should only be evaluated all together in the rule FinishAnalysis, so that they have the same consistent value across the entire query plan. To do that we mark current date-time functions as Unevaluable a while ago. Unfortunately, there are still some places that have to evaluate expressions earlier, and the expression may contain current date-time functions. #50800 is one such example.

I think the move to mark these functions as Unevaluable is too aggressive. This PR proposes to revert this restriction to avoid potential regressions. I think the better way forward is to introduce query context to pre-evaluate these current datetime values, and the expressions just get values from the query context and makes sure it produces consistent values during the query life cycle.

Why are the changes needed?

to avoid potential regressions.

Does this PR introduce any user-facing change?

no

How was this patch tested?

existing tests

Was this patch authored or co-authored using generative AI tooling?

no

[cloud-fan]

cc @MaxGekk @wankunde

[MaxGekk]

I guess you need to adjust CurrentTime:

case class CurrentTime(child: Expression = Literal(TimeType.MICROS_PRECISION))
  extends UnaryExpression with FoldableUnevaluable with ImplicitCastInputTypes {