feat: openai files provider (#3946)

# What does this PR do?
- Adds OpenAI files provider 
- Note that file content retrieval is pretty limited by `purpose`
https://community.openai.com/t/file-uploads-error-why-can-t-i-download-files-with-purpose-user-data/1357013?utm_source=chatgpt.com

## Test Plan
Modify run yaml to use openai files provider:
```
  files:
  - provider_id: openai
    provider_type: remote::openai
    config:
      api_key: ${env.OPENAI_API_KEY:=}
      metadata_store:
        backend: sql_default
        table_name: openai_files_metadata

# Then run files tests
❯ uv run --no-sync ./scripts/integration-tests.sh --stack-config server:ci-tests --inference-mode replay --setup ollama --suite base --pattern test_files
```

docs/docs/providers/files/remote_openai.mdx

@@ -0,0 +1,27 @@
+---
+description: "OpenAI Files API provider for managing files through OpenAI's native file storage service."
+sidebar_label: Remote - Openai
+title: remote::openai
+---
+
+# remote::openai
+
+## Description
+
+OpenAI Files API provider for managing files through OpenAI's native file storage service.
+
+## Configuration
+
+| Field | Type | Required | Default | Description |
+|-------|------|----------|---------|-------------|
+| `api_key` | `<class 'str'>` | No |  | OpenAI API key for authentication |
+| `metadata_store` | `<class 'llama_stack.core.storage.datatypes.SqlStoreReference'>` | No |  | SQL store configuration for file metadata |
+
+## Sample Configuration
+
+```yaml
+api_key: ${env.OPENAI_API_KEY}
+metadata_store:
+  table_name: openai_files_metadata
+  backend: sql_default
+```