Add MD5-Header option to CLDUploadRequestParams

Using OPTIONS, you can see that Cloudinary accepts `Content-MD5` as a header when uploading a file. Currently, there is no way to add this or any other headers to a `CLDUploadRequest`. The `Content-MD5` header seems like an important header to allow when uploading from iOS. The [Cloudinary Rails/Active storage integration](https://github.com/cloudinary/cloudinary_gem/blob/master/lib/active_storage/service/cloudinary_service.rb#L29) requires the MD5 be present, so this would bring the iOS client up to parity with the Ruby gem.