Supporting object store FFI

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

The use case here would be to be able to dynamically load libraries with custom object store implementations outside of s3/azure/gcp etc in libraries like polars in which there’s no way currently to register a new object store.

**Describe the solution you'd like**

An object store FFI would be necessary due to Rust’s unstable ABI.

**Describe alternatives you've considered**

**Additional context**

Raised an issue in Polars https://github.com/pola-rs/polars/issues/20568

Start by reading the linked Polars issue #20568 and this issue's discussion to understand the requested object-store use case and constraints. The issue does not name repository files or tests; done would require an agreed design for an object-store FFI that supports dynamically loaded custom implementations despite Rust's unstable ABI.

Written by the indexing model from the issue text.