Allow a type caster to be given to the `Arel::Table` object

This will allow most consuming code to avoid the deprecation introduced
in 008445d6fd5f825d9b445ac75a7be67f0f7ab52c. The only code which will be
affected is code that is building the `Arel::Table` object manually,
rather than calling `arel_table` on an Active Record class. Hopefully
this case will be rare enough that we don't need to introduce any
additional APIs to work around it.