Add BoundExpression

[Fokko]

Where #58 adds the unbound expressions, I think it would be valuable also to add the BoundExpressions. When an expression is passed in from the Query Engine, we need to bind it to the schema(s) of the Iceberg metadata. This converts an UnboundExpression to a BoundExpression where the unbound one references a field using a name, and the bound one references a field using its ID.