Packaging and optional dependencies #96

Summary
Base install pulls large cloud SDKs by default. Make heavy dependencies optional and modernize packaging.

Proposed changes

• Move cloud providers to extras: struct[s3], struct[gcs], struct[cloud].
• Pin compatible dependency versions for reproducibility.
• Adopt pyproject.toml (PEP 517/518); keep setup.py as shim if needed.
• Update docs/installation.md for extras and pinned installs.

Files

• requirements.txt -> optional extras in pyproject.toml
• setup.py / pyproject.toml
• docs/installation.md

Acceptance criteria

• pip install struct installs a lean base without boto3/GCS by default.
• pip install struct[s3] and struct[gcs] enable respective features.
• CI builds and tests pass with new packaging.