Refactor COPY TO to use format callback functions.

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

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

Additionally, 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 <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Reviewed-by: Andres Freund <[email protected]>
Reviewed-by: Tomas Vondra <[email protected]>
Reviewed-by: Junwang Zhao <[email protected]>
Discussion: https://postgr.es/m/[email protected]

Author: MasahikoSawada