Enable Python Client to work with non-AWS object stores (endpoint_override)

[joshuarobinson]

Using a non-AWS object store (e.g., Pure FlashBlade, Minio, etc) requires setting an endpoint_url, so that boto3 knows how to find the object store.

Note that Spark, Trino, et al provides these overrides already, so it's just the direct library that needs this support. (Perhaps Java does too need this? I'm only using python at the moment)

[jun-he]

Update the issue with python label and milestone.

[rdblue]

This is related to #5332. We should make sure this is possible with the new Python lib.

[joshuarobinson]

This will be fixed for PyArrow by #5747

[Fokko]

#5747 is in. Thanks for working on this @joshuarobinson, much appreciated 👍🏻