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`

llama_stack/providers/registry/files.py

@@ -5,9 +5,11 @@
 # the root directory of this source tree.
 
 from llama_stack.providers.datatypes import (
+    AdapterSpec,
     Api,
     InlineProviderSpec,
     ProviderSpec,
+    remote_provider_spec,
 )
 from llama_stack.providers.utils.sqlstore.sqlstore import sql_store_pip_packages
 
@@ -23,4 +25,14 @@ def available_providers() -> list[ProviderSpec]:
             config_class="llama_stack.providers.inline.files.localfs.config.LocalfsFilesImplConfig",
             description="Local filesystem-based file storage provider for managing files and documents locally.",
         ),
+        remote_provider_spec(
+            api=Api.files,
+            adapter=AdapterSpec(
+                adapter_type="s3",
+                pip_packages=["boto3"] + sql_store_pip_packages,
+                module="llama_stack.providers.remote.files.s3",
+                config_class="llama_stack.providers.remote.files.s3.config.S3FilesImplConfig",
+                description="AWS S3-based file storage provider for scalable cloud file management with metadata persistence.",
+            ),
+        ),
     ]