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

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

CopyReadAttributes{CSV,Text}() are used to parse lines for text and CSV
format.  This reduces the number of "if" branches that need to be
checked when parsing fields in CSV and text mode when dealing with a
COPY FROM, something that can become more noticeable with more
attributes and more lines to process.

Extracted from a larger patch by the same author.

Author: Sutou Kouhei
Discussion: https://postgr.es/m/20231204.153548.2126325458835528809.kou@clear-code.com