Deprecate duplicated column names in Table definition

Deprecate duplicated column names in Table definition

The :class:`_schema.Table` class now raises a deprecation warning
when columns with the same name are defined. To replace a column a new
parameter :paramref:`_schema.Table.append_column.replace_existing` was
added to the :meth:`_schema.Table.append_column` method.

The :meth:`_expression.ColumnCollection.contains_column` will now
raises an error when called with a string, suggesting the caller
to use ``in`` instead.

Co-authored-by: Federico Caselli <cfederico87@gmail.com>
Change-Id: I1d58c8ebe081079cb669e7ead60886ffc1b1a7f5