[Data] Add support for objects to Arrow blocks

[terraflops1048576]

Why are these changes needed?

Currently, Ray does not support blocks/batches with objects and multi-dimensional arrays in different columns. This causes Ray Data to throw exceptions when these are provided because:

1. Since there's an arbitrary object in the batch, the Arrow block format fails with ArrowNotImplemented with dtype 17. This falls back to return pd.DataFrame(dict(batch)) in BlockAccessor.batch_to_block.
2. However, this particular DataFrame constructor does not support columns with numpy.ndarray objects, so it throws the exception listed in the linked issue.

This change enables Python object storage in the Arrow blocks by defining an Arrow extension type that simply represents the Python objects as a variable-sized large binary. I suppose the alleged performance benefits listed in the comments are an extra benefit.

I'm not sure that this is the correct approach or that I've properly patched all of the places, so some help would be appreciated!

Related issue number

Resolves #45235

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[anyscalesam]

@terraflops1048576 this could be a big contribution but we want to think a little deeper about this with you to properly codevelop this; can you reach out to me on Ray Slack so we can setup some time to discuss further.

My handle is "Sam (Ray Team)"

We should get a formal REP in ray-enhancements for this as well: https://github.com/ray-project/enhancements

cc @bveeramani