SQLServer enable filter

[AdRiley]

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[radeusgd]

Overall shape looks great and aligned with what we discussed. Sorry if I'm asking for too many additional things if this is just an iterative improvement and supposed to be still improved in future iterations - I do understand some of my comments may just be things 'for later'.

For now:

1. I'd like to refactor some copy pasted code as it should increase clarity and maintainability:
   • the conversions in both directions - as this will make it much clearer where we are crossing boundaries between the boolean world and value world. Also BTW I'd suggest updating the docstring of _generate_expression with this 'two kinds of values' metaphor, as IMHO it is really helpful in understanding this whole ordeal.
   • minor thing but the application of metadata to the operation that was copied from Base_Generator should also be refactored IMHO.
2. I really don't understand why we have a separate top-level _generate_column function. As far as we discussed, it seems that there is nothing really special about a column - it is just a thing that expects a 'value level' expression and not booleans. So I'd assume I should just be able to call _generate_expression with generate_as_bool=False and it should work. If not, why?