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

[abonander]

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: apache/arrow-adbc#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.

[abonander]

It appears this is actually used in a few places in dbt-fusion: https://github.com/dbt-labs/dbt-fusion/blob/d9da8b70c2bef151d86ead3c42242754d0c998f0/crates/dbt-adapter/src/statement.rs#L121

The comment suggests that this can be called concurrently to running operations, which invokes undefined behavior due to aliasing &mut: https://doc.rust-lang.org/nomicon/aliasing.html

This should probably just change to be a no-op?

Interestingly, it's a todo!() in the DataFusion driver: https://github.com/apache/arrow-adbc/blob/2fa4f127738842b2b985da5d2c39dac8aaebe4ed/rust/driver/datafusion/src/lib.rs#L985-L987

It appears all Go implementations are hardwired (via the FFI) to return NotImplemented: https://github.com/apache/arrow-adbc/blob/2fa4f127738842b2b985da5d2c39dac8aaebe4ed/go/adbc/drivermgr/adbc_driver_manager.cc#L1272-L1275

There is no Statement.Cancel() in the Go API. Instead, there's an interesting note here on Statement.ExecuteQuery(): https://github.com/apache/arrow-adbc/blob/2fa4f127738842b2b985da5d2c39dac8aaebe4ed/go/adbc/adbc.go#L652-L653

I don't see anything to suggest something similar for the Rust API, so I have no idea if this behavior is expected of all drivers or just ones written in Go. It sounds to me like they haven't made up their minds how cancellation is actually supposed to work quite yet.

[abonander]

cc apache/arrow-adbc#3905