Support matching multipart request bodies while ignoring the multpart boundary key

The multipart boundary is often randomly generated and it's not important to match it exactly as long as the key and values of parts match.

cc @JoshLove-msft @scbedd

The issue names no files or tests. Start by locating the multipart request-body matching entry point, then verify how boundaries are parsed and compared. Done means multipart bodies match when part keys and values are equal even when the boundary differs.

Written by the indexing model from the issue text.