Enable a function to perform work during construction rather than each time it is invoked

[RCHowell]

Requested Solution/Feature

I was unable to find the old/existing issue related to this, but for some scalar functions it would be useful to pre-compile some inputs or memoize some state. Calcite supports this with FunctionContext on instantiation.

The original example was pre-compiling IonSchema so you can validate inputs without having to recompute the ISL every time.

https://github.com/apache/calcite/blob/main/core/src/main/java/org/apache/calcite/schema/FunctionContext.java