Support inspection of computed column

Support inspection of computed column

Added support for reflection of "computed" columns, which are now returned
as part of the structure returned by :meth:`.Inspector.get_columns`.
When reflecting full :class:`.Table` objects, computed columns will
be represented using the :class:`.Computed` construct.

Also improve the documentation in :meth:`Inspector.get_columns`, correctly
listing all the returned keys.

Fixes: #5063
Fixes: #4051
Closes: #5064
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5064
Pull-request-sha: ba00fc321ce468f8885aad23b3dd33c789e50fbe

Change-Id: I789986554fc8ac7f084270474d0b2c12046b1cc2
(cherry picked from commit 62b7dace0c1d03acf3224085d03a03684a969031)