Implement `Statement::cancel()` (or make it a no-op)

https://docs.rs/adbc_core/0.21.0/adbc_core/trait.Statement.html#tymethod.cancel

This is actually completely useless because executing a query requires `&mut self`, but possibly may be invoked via FFI: https://github.com/apache/arrow-adbc/issues/3454

This technically means it must be safe to call this concurrently to `Statement::execute()` despite both taking `&mut self`, which breaks Rust's soundness requirements for `&mut`. This is really problematic. But this is specifically a problem with the ADBC Rust API itself.

Blocked on the resolution of the linked issue. Until then, returning `NotImplemented` without touching `self` at all seems the safest.