Unbounded custom streams are not fully supported

[ianbotsf]

After the fix for #613 is in place, users will be able to provide custom/proxied streams but only when explicitly providing a non-null content length. There are seemingly-valid scenarios in which users may want to provide custom streams without an explicit content length, such as streaming from another source, building content on-the-fly, etc. We should come up with a way to support streams without an explicit content length, modify our stream type hierarchy to codify valid scenarios, improve error messaging to clarify expectations, or some combination thereof.

[aajtodd]

To add some additional context, I suspect this is less about how we have modeled the stream and more about how it is consumed in any given HTTP client engine implementation (e.g. conversion of null content-length to say transfer-encoding: chunked).

[aajtodd]

Also S3 requires Content-Length for PutObject and UploadPart (it should really be marked with requiresLength trait).

See aws/aws-sdk-net#1095 for relevant discussion around this limitation.