Beam SQL operator side condition validation is sensitive to optimization

Beam SQL converts Calcite's operators to Beam SQL operators post-optimization, when the final rels are being converted to a pipeline. This means that the optimization process can obscure the ability to validate things easily.

Specifically, for a `CEIL(date TO DAY)` we only support rounding to month or year (because of the underlying Calcite implementation. We want to be able to get the `DAY` and validate it before running. Today this is not validated properly because it is inconvenient.

Before optimization, we can check that the second operand is a symbol and see what it is. After optimization, the conversion to Calc hides it behind a LocalRef. We could look up the LocalRef but it would be cleaner to avoid evaluation-like logic during type checking/validation phase.

Imported from Jira [BEAM-4621](https://issues.apache.org/jira/browse/BEAM-4621). Original Jira may contain additional context.
Reported by: kenn.