Add ``omit_aliases`` in ``Enum``.

Add ``omit_aliases`` in ``Enum``.

Introduce a new parameter :paramref:`_types.Enum.omit_aliases` in
:class:`_types.Enum` type allow filtering aliases when using a pep435 Enum.
Previous versions of SQLAlchemy kept aliases in all cases, creating
database enum type with additional states, meaning that they were treated
as different values in the db. For backward compatibility this flag
defaults to ``False`` in the 1.4 series, but will be switched to ``True``
in a future version. A deprecation warning is raise if this flag is not
specified and the passed enum contains aliases.

Fixes: #6146
Change-Id: I547322ffa90d0273d91bb3bf8bfea6ec934d48b9