support "SELECT *" for ORM queries

support "SELECT *" for ORM queries

A :func:`_sql.select` construct that is passed a sole '*' argument for
``SELECT *``, either via string, :func:`_sql.text`, or
:func:`_sql.literal_column`, will be interpreted as a Core-level SQL
statement rather than as an ORM level statement. This is so that the ``*``,
when expanded to match any number of columns, will result in all columns
returned in the result. the ORM- level interpretation of
:func:`_sql.select` needs to know the names and types of all ORM columns up
front which can't be achieved when ``'*'`` is used.

If ``'*`` is used amongst other expressions simultaneously with an ORM
statement, an error is raised as this can't be interpreted correctly by the
ORM.

Fixes: #8235
Change-Id: Ic8e84491e14acdc8570704eadeaeaf6e16b1e870