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/header.rs

@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+use pyo3::prelude::*;
+use pyo3::types::PyDict;
+use std::collections::HashMap;
+
+/// A wrapper around a Python HeaderProvider that can be called from Rust
+pub struct PyHeaderProvider {
+    provider: Py<PyAny>,
+}
+
+impl Clone for PyHeaderProvider {
+    fn clone(&self) -> Self {
+        Python::with_gil(|py| Self {
+            provider: self.provider.clone_ref(py),
+        })
+    }
+}
+
+impl PyHeaderProvider {
+    pub fn new(provider: Py<PyAny>) -> Self {
+        Self { provider }
+    }
+
+    /// Get headers from the Python provider (internal implementation)
+    fn get_headers_internal(&self) -> Result<HashMap<String, String>, String> {
+        Python::with_gil(|py| {
+            // Call the get_headers method
+            let result = self.provider.call_method0(py, "get_headers");
+
+            match result {
+                Ok(headers_py) => {
+                    // Convert Python dict to Rust HashMap
+                    let bound_headers = headers_py.bind(py);
+                    let dict: &Bound<PyDict> = bound_headers.downcast().map_err(|e| {
+                        format!("HeaderProvider.get_headers must return a dict: {}", e)
+                    })?;
+
+                    let mut headers = HashMap::new();
+                    for (key, value) in dict {
+                        let key_str: String = key
+                            .extract()
+                            .map_err(|e| format!("Header key must be string: {}", e))?;
+                        let value_str: String = value
+                            .extract()
+                            .map_err(|e| format!("Header value must be string: {}", e))?;
+                        headers.insert(key_str, value_str);
+                    }
+                    Ok(headers)
+                }
+                Err(e) => Err(format!("Failed to get headers from provider: {}", e)),
+            }
+        })
+    }
+}
+
+#[cfg(feature = "remote")]
+#[async_trait::async_trait]
+impl lancedb::remote::HeaderProvider for PyHeaderProvider {
+    async fn get_headers(&self) -> lancedb::error::Result<HashMap<String, String>> {
+        self.get_headers_internal()
+            .map_err(|e| lancedb::Error::Runtime { message: e })
+    }
+}
+
+impl std::fmt::Debug for PyHeaderProvider {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "PyHeaderProvider")
+    }
+}