Add pgindent test to check if codebase is correctly formatted

Had some time to watch code run through an extensive test suite, so
thought I would propose this patch that is probably about 75% of the way
to the stated $subject. I had to add in a hack for Meson, and I couldn't
figure out a good hack for autotools.

I think a good solution would be to distribute pgindent and
pg_bsd_indent. At Neon, we are trying to format our extension code using
pgindent. I am sure there are other extension authors out there too that
format using pgindent. Distributing pg_bsd_indent and pgindent in the
postgresql-devel package would be a great help to those of us that
pgindent out of tree code. It would also have the added benefit of
adding the tools to $PREFIX/bin, which would make the test that I added
not need a hack to get the pg_bsd_indent executable.

--
Tristan Partin
Neon (https://neon.tech)