Draft: For travel agent, post raw inventory file as a multipart form

This changes how we perform metadata uploads to travel agent to post an existing metadata file as multipart form data rather than serializing it as a json object in the normal update request.

This is required so the backend server can deal with multi-gig metadata files without having to parse the JSON on the fly.

I am not good with golang so I cannot vouch for the quality of this code, it does however seem to work at least!