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.

(cherry picked from commit 868fd2d7)