feat: support per-request header override (#2631)

## Summary

This PR introduces a `HeaderProvider` which is called for all remote
HTTP calls to get the latest headers to inject. This is useful for
features like adding the latest auth tokens where the header provider
can auto-refresh tokens internally and each request always set the
refreshed token.

---------

Co-authored-by: Claude <noreply@anthropic.com>

python/src/lib.rs

@@ -20,6 +20,7 @@ use table::{
 pub mod arrow;
 pub mod connection;
 pub mod error;
+pub mod header;
 pub mod index;
 pub mod query;
 pub mod session;