Add BF member koel-like indentation checks to SanityCheck CI

Hi,

How about adding code indent checks (like what BF member koel has) to
the SanityCheck CI task? This helps catch indentation problems way
before things are committed so that developers can find them out in
their respective CI runs and lets developers learn the postgres code
indentation stuff. It also saves committers time - git log | grep
'koel' | wc -l gives me 11 commits and git log | grep 'indentation' |
wc -l gives me 97. Most, if not all of these commits went into fixing
code indentation problems that could have been reported a bit early
and fixed by developers/patch authors themselves.

Thoughts?

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com