[Patch] Omit virtual generated columns from test_decoding output

Hi Hackers,

Virtual generated columns are not stored on disk, so heap_getattr() in
tuple_to_stringinfo() always returned NULL for them, producing
misleading output such as

  table public.t: INSERT: a[integer]:1 b[integer]:10 c[integer]:null

even though the user could observe a non-null value via SELECT.  Stored
generated columns continue to be emitted as before because their values
do live in the heap tuple.

This matches the pgoutput's logicalrep_should_publish_column()
which never publishes virtual generated columns. Added a regression test.
Please find the patch attached.

Thanks,
Satya