Allow presigning object_store multipart uploads

[netthier]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

I want to be able to allow users to upload files to an object store using its multipart API without proxying the data through my server.

Describe the solution you'd like

Currently, signed URLs can be created through the Signer trait. It only has a single method which is highly generic across all request types. Not only does this mean that one cannot sign multipart requests, but it also means that one runs into problems such as #270 for PUT requests.
If the Signer API were made more granular, like the ObjectStore API, e.g. with separate methods for signing get, put, multipart, etc. requests, this could be solved at the cost of more boilerplate.

Describe alternatives you've considered

I can't think of good alternatives that would also allow for multipart uploads.