Backport of #48849. Update `.editorconfig` to make the Java settings the
default for all files, and then apply a 2-space indent to all `*.gradle`
files. Then reformat all the files.
Similarly to what has be done for Azure in #48636, this commit
adds a new :test:fixtures:gcs-fixture project which provides two
docker-compose based fixtures that emulate a Google Cloud
Storage service.
Some code has been extracted from existing tests and placed
into this new project so that it can be easily reused in other
projects.