[FFI] Move non-Arrow header functions out of arrow.rs into a common file

Follow-up from a review comment on #420: https://github.com/databricks/zerobus-sdk/pull/420#discussion_r3513892190 (by @davidtosovic-db):

Nit: This is not important and should not be done in this PR. All of these header functions don't really have to do anything with arrow. We should probably move them to the common file.

The header-related functions in rust/ffi/src/arrow.rs (around line 670) aren't Arrow-specific, so they'd be better placed in the shared/common FFI module rather than the Arrow file.

This was deliberately deferred out of #420 as a non-blocking nit. No behavior change intended — purely relocating the functions to improve organization.