feat(table): add namespace kwarg to ibis.table

[gforsyth]

A user asked about how to specify a table namespace when creating an unbound table.

The UnboundTable op does accept a namespace argument, but we don't expose that option in ibis/expr/api.py::table.

We will run afoul of the dual-definition of schema here, but if the user has to first construct the namespace object, at least that will be slightly separated.

Still not a good solution, really, to the schema/schema issue, but I don't know what is, short of breaking all ibis code.

[cpcloud]

Break everything, and cut over to version 100?