Add support for uploading a Stainless config file in addition to the OpenAPI spec file

[dgellow]

This pull request does three main changes:

1. The endpoint POST /api/spec has been updated to support a multipart/form-data body, making it possible to provide both the spec file and a Stainless config file. When the new input config_path is provided both the spec and config files will be uploaded, otherwise only the spec file will be uploaded.
2. While still supported, the use of S3 pre-signed URLs isn't required anymore. I removed that logic, the github action now relies on a single request POST /api/spec instead of the previous 3-steps dance.

[dgellow]

Tested this branch via a GitHub workflow, it works but we get a deprecation warning:

Run stainless-api/upload-openapi-spec-action@sam/sta-2798-update-upload-github-action-to-support-a-stainless-config-in
  with:
    stainless_api_key: ***
    input_path: specs/sink-openapi.yml
Uploading spec and config files...
(node:1383) [https://github.com/node-fetch/node-fetch/issues/1167] DeprecationWarning: form-data doesn't follow the spec and requires special treatment. Use alternative package
(Use `node --trace-deprecation ...` to show where the warning was created)

I will look into it.