feat: openai files provider

[ehhuang]

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

[raghotham]

Maybe not in this PR, but will be good to check to see if we can have multiple files providers in the same server.

[franciscojavierarceo]

nice

[ashwinb]

Maybe not in this PR, but will be good to check to see if we can have multiple files providers in the same server.

@raghotham @ehhuang yeah for that, we need to get a files router just like the vector_io router (if we don't have one.)