Open up SelectExpression construction

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To support the primitive collection and JSON querying work in 8.0, we needed to add some very scenario-specific constructors to SelectExpressions, e.g. accepting an arbitrary ordering column from the outside for OPENJSON/json_each), accepting an identifier from the outside, accepting a JsonQueryExpression...

We need to take a deeper look and open SelectExpression a bit so it can be properly constructed from the outside according to needs, rather than add a special constructor for each and every case. The problematic constructors are already marked as internal (though they're provider-facing!) so we can clean them up.

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Mostly done in #32877. Some details remain to be seen, and things won't be truly final until SelectExpression becomes immutable and all logic is removed from it; but for now this is enough.