Support Streams for attachments

[SamuelWeibel]

When transferring large attachments (e.g. files or HTTP POST payload in an ASP.NET application), it can be favorable to use Streams instead of creating a local byte[] with the entire content.

This PR introduces an abstraction to PutAttachmentRequest called (PutAttachmentRequestBase). I tried to implement it in a way so the API is backwards compatible; if you see any issues, please feel free to adjust it to your coding requirements.