Support onSendProgress for all request body types

[Tienisto]

Currently, the body type must be either HttpBody.bytes or HttpBody.stream (internally, HttpBody.bytes is converted to HttpBody.stream).

HttpBody.text and HttpBody.form might be convertable to HttpBody.stream too but then we introduce logic in Dart that might be inconsistent with reqwest.

HttpBody.multipart is an even bigger problem.

The best case would be if reqwest exposes an API for us: seanmonstar/reqwest#262

Edit: It seems that another library doesn't implement multipart either: algesten/ureq#698
So maybe we just ditch the "multipart" feature of reqwest, and build the byte stream ourself.