support create index on virtual generated column.

hi.
attached patch for implementing $subject feature.

* internally such index will be transformed into expression index.
  for example, an index on (b int GENERATED ALWAYS AS (a * 2) VIRTUAL) will be
  converted into an expression index on ((a * 2)).
* in pageinspect module, add some test to check the index content of
virtual generated column.
* primary key, unique index over virtual generated column are not supported.
  not sure they make sense or not.
* expression index and predicate index over virtual generated columns are
  currently not supported.
* virtual generated column can not be in "include column"
* all types of indexes are supported, and a hash index, gist test has
been added.
* To support ALTER TABLE SET EXPRESSION, in pg_index, we need to track
the original
  virtual generated column attribute number, so ALTER TABLE SET EXPRESSION can
  identify which index needs to be rebuilt.
* ALTER COLUMN SET DATA TYPE will also cause table rewrite, so we really
  need to track the virtual generated column attribute number that
index was built on.