Implement file streaming for httpclient's MultipartData (nim-lang#12982)

* Add `uploadFile` to POST files by streaming them

* Use constant for \c\L

* Formatting

* Remove uploadFile

* Implement MultipartData file streaming

* Remove unnecessary var annotations

* Call string on TaintedStrings

Fixes nim-lang#12789

* Move cl constant to httpcore

* Fix `request` inconsistencies

* Update documentaion

* Clean up

* Skip multipart formatting when there's 0 entries

* Remove extraneous `cl` from multipart formatting

* Update MultipartData `$` to match old behaviour

* Update comment

* Address comments