This patch adds a simple job that checks for the clang-format
conformance, similar to the Travis CI job did in commit 54273fdd
(greenplum-db/gpdb#10357).

I was not amused when I realized the gen_pipeline.py doesn't do the due
diligence of finding transitive dependencies (so the new job actually
already blocks release, but the script insists on stopping at immediate
blockers). But that's left for another day.