Add an easy-to-use API for creating a multipart/form-data request body and sending it. This API should allow for referencing one or more file streams as form parts. Allowing for files is tricky as described in #7.

There are a couple existing crates we could make use of:

• https://crates.io/crates/multipart
• https://crates.io/crates/multipart-async
• https://crates.io/crates/mime_multipart
• https://crates.io/crates/formdata

At first glance, none of these appear to support a fully async writing of forms though, except multipart-async which looks unstable, which is unfortunate.