Create workspace with core and util crates

[timsaucer]

Which issue does this PR close?

Closes #1395.

Rationale for this change

For our downstream users who are implementing FFI based extensions, there are a handful of useful utility functions
that we are currently requiring them to copy. This is a poor way to keep code up to date. This PR creates a utility crate that our downstream users can utilize. This also helps to ensure that as our interfaces get updated that those changes propagate properly to our downstream users.

Also this change ensures our example is on the same code version as the main repository.

What changes are included in this PR?

• Adds a cargo workspace
• Divides the code into three projects, core, util and the FFI example.
• Updates dependencies to all use the common workspace.

Are there any user-facing changes?

Yes, there is a slight change in that the utils module of datafusion-python has been removed. Instead users can now change their dependency to the new datafusion-python-util crate.

[timsaucer]

FYI @kevinjqliu

[davisp]

+1

Everything looks straightforward to me. I only had the one minor question on the removed cargo config.

[davisp]

Am I missing where these got moved to, or do we just not need them any more?

[timsaucer]

Removed because these were serving the same functionality as the pyo3_build_config::add_extension_module_link_args() in build.rs

[kevinjqliu]

speaking of build.rs, i was testing out this integration in iceberg-rust and claude mentioned that i need to create my own build.rs file for compatibility

is this expected?

[kevinjqliu]

LGTM, i built iceberg-rust using this branch (https://github.com/kevinjqliu/iceberg-rust/pull/8/files#diff-1a7b28a196f60421838fd52ca356647a835081dc3095f1b55d526c27580cb677R23)

one nit: do you prefer the crate to be named -util or -utils?