Spike: Direct Git LFS uploads

Currently it is not possible to directly upload LFS files to external object stores. Theoretically we can support this as git-lfs documents here.

I believe we can build the headers for the upload action and then add a new verify action to the /batch upload_actions method. In the verify action we can send a callback to our servers to confirm the file was uploaded and then create a background job to link it to the project.

This should help offload some processing which will likely resolve issues like I/O timeout when pushing LFS objects (#371908 - closed)

We need to first investigate how to implement so I'm creating this as a spike We should also document how the upload flow currently works.