ci: Prepare to make compute resources for CI configurable

cirrus-ci will soon restrict the amount of free resources every user gets (as
have many other CI providers). For most users of CI that should not be an
issue. But e.g. for cfbot it will be an issue.

To allow configuring different resources on a per-repository basis, introduce
infrastructure for overriding the task execution environment. Unfortunately
this is not entirely trivial, as yaml anchors have to be defined before their
use, and cirrus-ci only allows injecting additional contents at the end of
.cirrus.yml.

To deal with that, move the definition of the CI tasks to
.cirrus.tasks.yml. The main .cirrus.yml is loaded first, then, if defined, the
file referenced by the REPO_CI_CONFIG_GIT_URL variable, will be added,
followed by the contents of .cirrus.tasks.yml. That allows
REPO_CI_CONFIG_GIT_URL to override the yaml anchors defined in .cirrus.yml.

Unfortunately git's default merge / rebase strategy does not handle copied
files, just renamed ones. To avoid painful rebasing over this change, this
commit just renames .cirrus.yml to .cirrus.tasks.yml, without adding a new
.cirrus.yml. That's done in the followup commit, which moves the relevant
portion of .cirrus.tasks.yml to .cirrus.yml.  Until that is done,
REPO_CI_CONFIG_GIT_URL does not fully work.

The subsequent commit adds documentation for how to configure custom compute
resources to src/tools/ci/README

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/20230808021541.7lbzdefvma7qmn3w@awork3.anarazel.de
Backpatch: 15-, where CI support was added

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