ADBC FFI types and possible abstraction

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

Not sure if this should live here or apache/arrow-adbc. Probably depends on process we want for release.

It would be nice to have a library crate that contains the struct definitions for FFI structs and the error codes as an enum.

Additionally, it may be possible to create a module with a macro and traits that allows a developer to implement the ADBC API without having the interact directly with the FFI structs and write unsafe code. I'm prototyping such an abstraction right now.

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