pgsql: Re-export NextCopyFromRawFields() to copy.h.

Re-export NextCopyFromRawFields() to copy.h.

Commit 7717f630069 removed NextCopyFromRawFields() from copy.h. While
it was hoped that NextCopyFrom() could serve as an alternative,
certain use cases still require NextCopyFromRawFields(). For instance,
extensions like file_text_array_fdw, which process source data with an
unknown number of columns, rely on this function.

Per buildfarm member crake.

Reported-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Sutou Kouhei <kou@clear-code.com>
Discussion: https://postgr.es/m/5c7e1ac8-5083-4c08-af19-cb9ade2f16ce@dunslane.net

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8a1012b35d6123d8ee1818fce68f435459e297b7

Modified Files
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src/backend/commands/copyfromparse.c | 28 +++++++++++++++++++++++-----
src/include/commands/copy.h          |  2 ++
2 files changed, 25 insertions(+), 5 deletions(-)