feat: support cloud filesystem model paths

[goutamadwant]

Summary

Closes #148.

This change allows model hashing, signing, verification, and digest operations to read model artifacts from GCS and S3 URIs without staging the complete model on local disk.

The implementation:

• introduces a small URI-aware filesystem boundary using etils.epath;
• preserves existing local pathlib behavior;
• traverses remote directories without recursive glob assumptions;
• streams normal and BLAKE3 hashing in bounded chunks;
• uses seek/range reads for sharded hashing;
• adds optional gcs and s3 provider extras;
• keeps keys, certificates, trust configuration, and signature bundles local;
• documents provider setup and the tested support boundary.

The cloud path is covered with an isolated in-memory gs:// backend, including local/remote manifest equivalence, bounded reads, relative ignore paths, signing and verification, tamper detection, digest CLI behavior, BLAKE3, and sharded hashing.

Validation completed:

• Python 3.10-3.14: 208 tests passed per version, 1,040 total
• cloud-specific tests: 8 passed
• Ruff formatting and lint: passed
• Pytype: 54 sources checked with no errors
• wheel and source distribution builds: passed
• package metadata and clean archive validation: passed
• local CLI key/certificate and historical fixture tests: passed
• git diff --check: passed

Live cloud credentials were not exercised. External Fulcio signing was unavailable in the test environment because of a TLS WRONG_VERSION_NUMBER failure, and PKCS#11 integration requires SoftHSM, which was not installed.

Checklist

• All commits are signed-off, using DCO
• All new code has docstrings and type annotations
• New cloud filesystem behavior is covered by focused tests
• Public-facing changes are paired with documentation changes
• A release note has been added to CHANGELOG.md