Add more nesting features to add_cte()

Add more nesting features to add_cte()

Added new parameter :paramref:`.HasCTE.add_cte.nest_here` to
:meth:`.HasCTE.add_cte` which will "nest" a given :class:`.CTE` at the
level of the parent statement. This parameter is equivalent to using the
:paramref:`.HasCTE.cte.nesting` parameter, but may be more intuitive in
some scenarios as it allows the nesting attribute to be set simultaneously
along with the explicit level of the CTE.

The :meth:`.HasCTE.add_cte` method also accepts multiple CTE objects.

Fixes: #7759
Change-Id: I263c015f5a3f452cb54819aee12bc9bf2953a7bb