Implement `OptionStatement::{Progress, MaxProgress}`

I'm not sure how these are actually supposed to work: https://docs.rs/adbc_core/latest/adbc_core/options/enum.OptionStatement.html#variant.Progress

> This represents the progress of execution, not of consumption (i.e., it is independent of how much of the result set has been read by the client).

To me, this would mean getting the progress *while the statement is blocked on execution*, e.g. in `Statement::execute()` or `Statement::execute_update()`, which would be another mutable aliasing issue like with `Statement::cancel()` (#20).

I've asked folks from Columnar for clarification.

Start with the OptionStatement::Progress and MaxProgress documentation, then inspect the Statement::execute() and execute_update() entry points and the cancellation work in issue #20. Resolve the intended execution-progress semantics with the referenced Columnar clarification, including how mutable aliasing is handled. Done means both options have an agreed meaning and an implementation that exposes progress without conflicting with statement execution.

Written by the indexing model from the issue text.