Feature request: Enable explicit multipart requests in http processor, similar to http_client output

The http processor will dispatch multipart requests with parts formed from the message batch, as is usual in Benthos.

However, the processor does not afford much control over the multipart form, which makes it hard to dispatch arbitrary multipart-form requests.

The http output supports an experimental multipart feature, which solves this exact problem. Porting this logic to the http processor, would be a small step for Benthos, but a giant leap for humankind.

Compare the http processor's multipart request handling with the http output's experimental multipart feature. Trace how form control is exposed in the output and determine what equivalent behavior the processor needs. Done means arbitrary multipart-form requests can be dispatched through the processor while preserving its existing message-batch behavior.