Improve COPY TO performance when server and client encodings match

This commit fixes an oversight introduced in c61a2f58418e, where COPY TO
would attempt to do encoding conversions even if the encodings of the
client and the server matched for multi-byte encodings.  All conversions
go through pg_any_to_server() that makes the conversion a no-op when the
encodings of the client and the server match, even for multi-byte
encodings.

The logic was fine, but setting CopyToStateData->need_transcoding would
cause strlen() to be called for nothing for each attribute of all the
rows copied, and that was showing high in some profiles (more attributes
make that easier to reach).  This change improves the runtime of some
worst-case COPY TO queries by 15%~ (number present at least here).

This is a performance improvement, so no backpatch is done out of
caution as this is not a regression.

Reported-by: Andres Freund
Analyzed-by: Andres Freund
Author: Michael Paquier
Reviewed-by: Heikki Linnakangas
Discussion: https://postgr.es/m/20240206020504.edijzczkgd25ek6z@awork3.anarazel.de

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