Support adding PUT headers for object storage from Rails

As revealed in https://gitlab.com/gitlab-org/gitlab-ce/issues/49957, Rails
generates a signed URL with a fixed HTTP header with `Content-Type:
application/octet-stream`. However, if we change or remove that for
some reason in Workhorse, this breaks the upload with a 403 Unauthorized because
the signed URL is not valid.

We can make this more robust by doing the following:

1. In the `/uploads/authorize` request, Rails can return a `StoreHeaders` key-value
pair in the JSON response containing the required headers that the PUT
request must include.
2. Use those HTTP headers if that value is present.
3. For backwards compatibility, if that key is not present, default to
the old behavior of sending the fixed `Content-Type` header.