This commit adds the Concourse build infrastructure under the "ci/"
folder. It contains:

* "images/", the definitions of container images used in the pipeline
* "pipeline.yml" the build pipeline definition
* "parameters.yml" the parameters for the pipeline definition
* "tasks/" the tasks used by the build pipeline
* "scripts/" scripts used by pipeline tasks

This first iteration of the Concourse pipeline builds CI container
images for JDK8, JDK11, JDK14 and JDK15.
These images are used for building and publishing the master branch of
Spring Framework using JDK8. Other JDK variants are sanity checks and
don't actually publish the produced artifacts.

See gh-22490