Support dict encoded structs in `get_field`

[brancz]

Is your feature request related to a problem or challenge?

Our schema contains dictionary encoded structs, and when we want to write a statement like:

SELECT mystruct.myfield FROM mytable;

This errors.

Describe the solution you'd like

I want to be able to query my schema, to do that we need support for resolving dict accesses in get_field.

Describe alternatives you've considered

Casting the column to its non-dict encoded self, this is what we do right now just to get it to work, but obviously that's a bad idea for performance.

Additional context

Already have the implementation, this is just for tracking.