Expose `ExecutionPlan` statistics across the FFI boundary

[mailmindlin]

Is your feature request related to a problem or challenge?

ExecutionPlan::partition_statistics() is not currently reachable through datafusion-ffi: FFI_ExecutionPlan exposes most of the trait (properties, children, with_new_children, name, execute, repartitioned, and soon metrics) but has no function pointer for partition_statistics(), so ForeignExecutionPlan::partition_statistics() falls through to the trait default implementation and always returns Statistics::new_unknown().

Describe the solution you'd like

Expose partition_statistics() across the FFI boundary by serializing the Statistics struct using protobuf, similar to how filters are currently handled. This avoids creating a parallel type system.

Describe alternatives you've considered

Create a mapping/conversion layer from the current Statistics type to FFI-safe types. Rejected: it requires defining many type variants and protobuf serialization results in much less code.

Additional context

I have a PR already authored.

This will be a breaking change requiring a major version bump.