Refactor COPY FROM to use format callback functions.

This commit introduces a new CopyFromRoutine struct, which is a set of
callback routines to read tuples in a specific format. It also makes
COPY FROM with the existing formats (text, CSV, and binary) utilize
these format callbacks.

This change is a preliminary step towards making the COPY FROM command
extensible in terms of input formats.

Similar to 2e4127b6d2d, this refactoring contributes to a performance
improvement by reducing the number of "if" branches that need to be
checked on a per-row basis when sending field representations in text
or CSV mode. The performance benchmark results showed ~5% performance
gain in text or CSV mode.

Author: Sutou Kouhei <kou@clear-code.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Discussion: https://postgr.es/m/20231204.153548.2126325458835528809.kou@clear-code.com

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