Remove typedefs (e.g. ExecutionPlanFilterPushdownResult)

[alamb]

Targets apache#15566

Here is a proposal to simplify the APIs.

In my opinion a typedef in Rust just adds cognitive overhead and makes the code harder to read. If we need a new type we should make a real struct with documentation, etc. A typedef just adds an extra layer of indirection.

I also think this change makes the code simpler

Both of these are subjective opinions

[adriangb]

Don't fully agree but also think it's a super minor thing, let's go with your version!