Changes made by commit 02ddd499 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