Implement multipart_inject() or mime_inject()

[lcsmuller]

For sending a multipart/form-data content type we are required to send a body in the appropriate multipart format type (See here). Currently the temporary solution is to use curl_mime type, which can be used with some related functions for creating the multipart body. Unfortunately we can't access the multipart content itself and have to expose curl functions to the library developers. (See discord-public-channel.cpp:210 for example)

The inclusion of multipart_inject()/mime_inject() could provide a way to create multipart type body which can then be easily accessed via struct sized_buffer req_body

[mlite]

We have a working solution, the need of this is low.