fix(go/adbc/sqldriver): fix handling of decimal types

[zeroshade]

fixes #969

[lidavidm]

Can we fix the issue where the user didn't get an error for an unrecognized type?

[zeroshade]

sure, let me dig a bit further into that

[zeroshade]

So, as far as I can tell from my testing, the user didn't get an error because they didn't call the .Err method on the rows object to check, as per the usual pattern. I can't see any other situation, nor replicate a situation where the .Err doesn't return the error. So i don't think there's an issue to be fixed there.

[lidavidm]

Should've used Rust 😉

LGTM then. Thanks!