Allow specifying Content-Type when signing object_store PUT requests

[netthier]

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

I want to create presigned PUT URLs so that users can upload content without having to proxy it through my server. This is mostly already possible with the current Signer trait, but providers such as AWS require the Content-Type header, if set, to be included in the signature (https://docs.aws.amazon.com/IAM/latest/UserGuide/create-signed-request.html).
This means it's currently not possible to generate presigned URLs allowing users to set the Content-Type header while uploading (unless it is application/octet-stream, which I believe is the default).

Describe the solution you'd like

I'd like to be able to optionally specify the Content-Type when signing a PUT request, such that users can set it themselves when using it.

Describe alternatives you've considered

I'm not aware of any alternatives, except using additional crates.