feat: Add S3 Files Provider implementation

Implements a complete S3-based file storage provider for Llama Stack with:

Core Implementation:
- S3FilesImpl class with full OpenAI Files API compatibility
- Support for file upload, download, listing, deletion operations
- Sqlite-based metadata storage for fast queries and API compliance
- Configurable S3 endpoints (AWS, MinIO, LocalStack support)

Key Features:
- Automatic S3 bucket creation and management
- Metadata persistence
- Proper error handling for S3 connectivity and permissions

Dependencies:
- Adds boto3 for AWS S3 integration
- Adds moto[s3] for testing infrastructure

Testing:

 Unit: `./scripts/unit-tests.sh tests/unit/files tests/unit/providers/files`

 Integration:

  Start MinIO: `podman run --rm -it -p 9000:9000 minio/minio server /data`

  Start stack w/ S3 provider: `S3_ENDPOINT_URL=http://localhost:9000 AWS_ACCESS_KEY_ID=minioadmin AWS_SECRET_ACCESS_KEY=minioadmin S3_BUCKET_NAME=llama-stack-files uv run llama stack build --image-type venv --providers files=remote::s3 --run`

  Run integration tests: `./scripts/integration-tests.sh --stack-config http://localhost:8321 --provider ollama --test-subdirs files`

pyproject.toml

@@ -121,6 +121,7 @@ test = [
     "pymilvus>=2.5.12",
     "milvus-lite>=2.5.0",
     "weaviate-client>=4.16.4",
+    "moto[s3]>=5.1.10",
 ]
 docs = [
     "setuptools",