feat: add native OAuth/OIDC authentication support

Add OAuthConfig and OAuthHeaderProvider to the Rust core with support
for five OAuth flows: ClientCredentials, AuthorizationCodePKCE,
DeviceCode, AzureManagedIdentity, and WorkloadIdentity. Token
acquisition and auto-refresh happen entirely in Rust.

Python and TypeScript expose OAuthConfig as a plain config object that
maps to the Rust header provider via FFI — no dynamic callbacks cross
the language boundary.

ConnectBuilder gains an oauth_config() method that replaces the API key
requirement when OAuth is configured.

python/tests/test_oauth.py

@@ -0,0 +1,33 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+import importlib.util
+import sys
+from pathlib import Path
+
+
+def _load_oauth_module():
+    oauth_path = (
+        Path(__file__).parents[1] / "python" / "lancedb" / "remote" / "oauth.py"
+    )
+    spec = importlib.util.spec_from_file_location("lancedb_remote_oauth", oauth_path)
+    module = importlib.util.module_from_spec(spec)
+    assert spec.loader is not None
+    sys.modules[spec.name] = module
+    spec.loader.exec_module(module)
+    return module
+
+
+def test_oauth_config_repr_redacts_client_secret():
+    oauth = _load_oauth_module()
+
+    config = oauth.OAuthConfig(
+        issuer_url="https://issuer.example.com",
+        client_id="client-id",
+        scopes=["scope"],
+        client_secret="super-secret",
+    )
+
+    rendered = repr(config)
+    assert "super-secret" not in rendered
+    assert "client_secret" not in rendered