[BEAM-10925] Create ZetaSQL-specific subclass of ScalarFunctionImpl t…

[ibzib]

…hat knows which function group it belongs to.

This will be important for differentiating ZetaSQL built-in functions from UDFs.

Also resolves the nullability error in findMethod.

R: @amaliujia @kennknowles

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[amaliujia]

for these ZETASQL_FUNCTION_GROUP_NAME, eventually I think we will migrate it off from createUdfOperator.

[apilloud]

I'm a little concerned about making createUdafOperator/createUdfOperator public. Otherwise LGTM

[apilloud]

nit: It seems like SqlAnalyzer.ZETASQL_FUNCTION_GROUP_NAME is the only thing ever passed to funGroup here and below. Should it really be an argument?

[ibzib]

The full implementation uses the group name to distinguish built-in functions from user-defined functions. (Whether that's a good idea or not is up for debate.)

[apilloud]

This (and createUdfOperator) is intended to be a (slightly hacky) helper function for implementing ZetaSQL operators via the UDF interface. Why do you need to make them public?

[ibzib]

In the original PR, I used createUdfOperator in ExpressionConverter, and createUdafOperator in AggregateScanConverter. So they can be package-private instead of public.