[Feature request] Multiple --data options without &-symbol

Hello!

Currently, when specifying more than one --data options with curl, the data pieces specified are merged with a separating &-symbol. 

While creating a request for a 3GPP API, I had the need of mixing JSON data with binary data in a multipart/related content-type request and each time a --data option is used, the data is joined by the & symbol, making the request useless.

Example:
curl --http2-prior-knowledge \
  --request POST \
  --header "Content-Type: multipart/related; boundary=‘=-myboundary=='" \
  --data-binary $'--=-myboundary==\r\nContent-Type: application/json\r\n\r\n' \
  --data @data.json \
  --data-binary $'\r\n--=-myboundary==\r\nContent-Type: application/vnd.3gpp.5gnas\r\n\r\n' \
  --data-binary @nas_pdu.bin \
  --data-binary $'\r\n--=-myboundary==--\r\n' \
webserver:80/resource

I thought something like this could be useful in some environments, like specifying the request like this and not having the data parts merged with & by adding a special option like --avoid-merge.

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