feat: Add S3 Files Provider (#3202)

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

@@ -98,6 +98,7 @@ unit = [
     "together",
     "coverage",
     "chromadb>=1.0.15",
+    "moto[s3]>=5.1.10",
 ]
 # These are the core dependencies required for running integration tests. They are shared across all
 # providers. If a provider requires additional dependencies, please add them to your environment