Send required object storage PUT headers in /uploads/authorize API

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.

See https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/297 as well.

changelogs/unreleased/sh-send-put-headers-object-storage.yml

+---
+title: Send back required object storage PUT headers in /uploads/authorize API
+merge_request: 21319
+author:
+type: changed

lib/object_storage/direct_upload.rb

@@ -41,7 +41,9 @@ module ObjectStorage
         GetURL: get_url,
         StoreURL: store_url,
         DeleteURL: delete_url,
-        MultipartUpload: multipart_upload_hash
+        MultipartUpload: multipart_upload_hash,
+        CustomPutHeaders: true,
+        PutHeaders: upload_options
       }.compact
     end
 

spec/lib/object_storage/direct_upload_spec.rb

@@ -61,6 +61,8 @@ describe ObjectStorage::DirectUpload do
         expect(subject[:GetURL]).to start_with(storage_url)
         expect(subject[:StoreURL]).to start_with(storage_url)
         expect(subject[:DeleteURL]).to start_with(storage_url)
+        expect(subject[:CustomPutHeaders]).to be_truthy
+        expect(subject[:PutHeaders]).to eq({ 'Content-Type' => 'application/octet-stream' })
       end
     end