pgsql: Provide an extra-float-digits setting for pg_dump / pg_dumpall

Provide an extra-float-digits setting for pg_dump / pg_dumpall

Changes made by commit 02ddd49 mean that dumps made against pre version
12 instances are no longer comparable with those made against version 12
or later instances. This makes cross-version upgrade testing fail in the
buildfarm. Experimentation has shown that the error is cured if the
dumps are made when extra_float_digits is set to 0. Hence this patch
allows for it to be explicitly set rather than relying on pg_dump's
builtin default (3 in almost all cases). This feature might have other
uses, but should not normally be used.

Discussion: https://postgr.es/m/c76f7051-8fd3-ec10-7579-1f8842305b85@2ndQuadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/af25bc03e17eb0aba195e42506b1a15f47178e44

Modified Files
--------------
doc/src/sgml/ref/pg_dump.sgml    | 11 +++++++++++
doc/src/sgml/ref/pg_dumpall.sgml | 11 +++++++++++
src/bin/pg_dump/pg_dump.c        | 31 ++++++++++++++++++++++++++++---
src/bin/pg_dump/pg_dumpall.c     |  7 +++++++
4 files changed, 57 insertions(+), 3 deletions(-)