Revert "Refactor CopyReadAttributes{CSV,Text}() to use a callback in COPY FROM"

This reverts commit 95fb5b49024, for reasons similar to what led to
1aa8324b81fa.  In this case, the callback was called once per row, which
is less worse than the previous callback introduced for COPY TO called
once per argument for each row, still the patch set discussed to plug in
custom routines to the COPY paths would be able to know which subroutine
to use depending on its CopyFromState, so this led to a suboptimal
approach at the end.

For now, this part is reverted to consider better which approach to use.

Discussion: https://postgr.es/m/20240206014125.qofww7ew3dx3v3uk@awork3.anarazel.de

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