[ntuple] Add S3 anchor and its tests for S3 backend

[JasMehta08]

This Pull request:

(Is a part of the GSoC 2026 project S3 Backend for RNTuple.)

Creates the anchor for the S3 backend.

Changes or fixes:

• Adds S3NTupleAnchor struct, using JSON to store the anchor data.
• Adds ROOT_BUILD_OPTION(s3 OFF "Enable RNTuple support for S3-compatible object storage")to conditionally compile.
• Adds tests that cover round-trip, version guard, malformed input, type mismatches, boundary values, and forward compatibility.

Checklist:

• tested changes locally

[JasMehta08]

@jblomer

I have made the changes so that the build for S3 backend only occurs when curl is present, and removed the s3 flag.