[#188]:svarga:feature, add HDF5 S3 / ROS3 VFD support

[steven-varga]

Summary

• Adds h5::ros3 / h5::fapl_ros3 property class wrapping H5Pset_fapl_ros3 — the HDF5 Read-Only S3 VFD — guarded by H5_HAVE_ROS3_VFD
• Supports v1 FAPL (long-term credentials, HDF5 ≥ 1.10.6) and v2 FAPL (session token / STS, HDF5 ≥ 1.12.1)
• CMake detects ROS3 availability via HDF5 pubconf header; exposes h5::have_ros3_vfd constexpr bool
• Deprecates stale fapl_rest_vol / kita comment (pre-1.12 REST VOL API now replaced by H5Pset_vol)
• Offline doctest suite (test/H5FDros3.cpp) + opt-in live network test against verified public bucket (-DH5CPP_NETWORK_TESTS=ON)
• Example (examples/s3/s3.cpp) demonstrates all three credential modes using h5::ls for directory listing
• User guide (docs/s3.md) covers prerequisites, API reference, and limitations

Test plan

• cmake -S . -B build && cmake --build build --parallel && ctest --test-dir build --output-on-failure passes (offline tests)
• cmake -DH5CPP_NETWORK_TESTS=ON and run test-h5fdros3_network with outbound S3 access to confirm live open of s3://rhdf5-public/h5ex_t_array.h5
• Confirm cmake status line prints H5CPP: ROS3 VFD detected on a standard Ubuntu HDF5 install
• Confirm build succeeds with H5CPP_NETWORK_TESTS=OFF (default) on CI

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