[Feature Request]: HttpPost / Send multiple part when multipart is set

### What would you like to happen?

The httppost transfomer is configurable to send the data as multipart.
However, it allows just a part to be set.

The feature request is about allowing multiple part to be defined.

### Issue Priority

Priority: 3

### Issue Component

Component: Transforms

Start by locating the HttpPost transformer in the Transforms component and read how multipart data is currently configured. The change is complete when the transformer accepts multiple defined parts and sends them together as multipart; the issue names no tests, so identify the relevant test coverage while tracing the existing behavior.

Written by the indexing model from the issue text.