S3 Backend for RNTuple

RNTuple is ROOT's next-generation columnar data format for High Energy Physics. Its storage layer is decoupled from its format layer, enabling new backends. This project implements a S3 storage backend for RNTuple, allowing datasets to be written to and read from S3-compatible object stores such as CERN's Ceph S3 and AWS S3. The approach introduces a new S3 locator type with two addressing modes: packed pages with multi-range GET for servers that support it, and one-page-per-object with concurrent GETs for those that do not. A JSON anchor at the base URL provides the entry point with a templated URL scheme for forward compatibility. The implementation adds RPageSinkS3 and RPageSourceS3 classes using the existing RCurlConnection as the transport layer, extended with PUT support and concurrent I/O via curl multi handles. Deliverables include the complete write path (Phase 1-2), complete read path with optimized cluster loading (Phase 3-4), RCurlConnection extensions for PUT and concurrent I/O, and a comprehensive test suite covering wire-level tests, locator round-trips, integration tests against MinIO and CERN Ceph S3, and performance benchmarks.