rust: split workspaces so that dependencies of adbc_core aren't tied to what the datafusion driver requires

[felipecrv]

What feature or improvement would you like to see?

As discussed in #2525 (comment)

[paleolimbot]

For what it's worth, I asked at the DataFusion community call and it seems this type of dependency weirdness between Arrow, Data Fusion, and other tooling happens everywhere. When adbc_core and the datafusion driver are a bit more fleshed out I think we can approach those communities to move the projects to minimize the weirdness.