Support file upload

[masci]

At the moment it's extremely hard to use Hayhooks for indexing pipelines, as they either accept:

• a file path, which is a no-go as it's relative to the filesystem where Hayhooks is running
• a bytestream, which is hard to manage see Sending a file via ByteStream #19

Since Hayhooks is in control of the request payload for pipeline endpoints, one possible solution might be accepting multipart form data whenever the input of a pipeline is of type path or bytestream. Hayhooks would receive the file and take care of temporarily storing it server-side, or passing bytes on-the-fly to the pipeline.