pgsql: ci: Add continuous integration for github repositories via cirru

ci: Add continuous integration for github repositories via cirrus-ci.

Currently FreeBSD, Linux, macOS and Windows (Visual Studio) are tested.

The main goal of this integration is to make it easier to test in-development
patches across multiple platforms. This includes improving the testing done
automatically by cfbot [1] for commitfest entries. It is *not* the goal to
supersede the buildfarm.

cirrus-ci [2] was chosen because it was already in use for cfbot, allows using
full VMs, has good OS coverage and allows accessing the full test results
without authentication (like a github account). It might be worth adding
support for further CI providers, particularly ones supporting other git
forges, in the future.

To keep CI times tolerable, most platforms use pre-generated images. Some
platforms use containers, others use full VMs.

For instructions on how to enable the CI integration in a repository and
further details, see src/tools/ci/README

[1] http://cfbot.cputube.org/
[2] https://cirrus-ci.org/

Author: Andres Freund <andres(at)anarazel(dot)de>
Author: Thomas Munro <tmunro(at)postgresql(dot)org>
Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-By: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-By: Justin Pryzby <pryzby(at)telsasoft(dot)com>
Reviewed-By: Thomas Munro <tmunro(at)postgresql(dot)org>
Reviewed-By: Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>
Discussion: https://postgr.es/m/20211001222752.wrz7erzh4cajvgp6@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/93d97349461347d952e8cebdf62f5aa84b4bd20a

Modified Files
--------------
.cirrus.yml | 547 ++++++++++++++++++++++++++++++++
src/tools/ci/README | 63 ++++
src/tools/ci/cores_backtrace.sh | 50 +++
src/tools/ci/gcp_freebsd_repartition.sh | 28 ++
src/tools/ci/pg_ci_base.conf | 14 +
src/tools/ci/windows_build_config.pl | 13 +
6 files changed, 715 insertions(+)