postgresql `pair` function

Sometimes in postgresql you need to pass a pair of value to a function.

The example I have in mind is something like `array_agg ((foo.bar, foo.baz))` (the double parenthesis is required).

It would be nice to have a `pair` function with signature `SqlExpr (Value a) -> SqlExpr (Value b) -> SqlExpr (Value (a, b))` to construct such values

No file or test is named. Start by locating existing SqlExpr helper functions and the PostgreSQL expression rendering or tuple-handling entry points, then compare their patterns with the requested pair signature and array_agg example. Done means the helper constructs the requested PostgreSQL pair value and has coverage for the shown use case.

Written by the indexing model from the issue text.