Support schema for multipart requests

[adamchalmers]

Currently, multipart bodies always create an API schema like this:

"content": {
  "multipart/form-data": {
    "schema": {
      "type": "string",
      "format": "binary"
    }
  }
}

In my dropshot API, my request handler accepts this multipart body and then extracts parts from it. I expect some parts to conform to some schema. e.g. "first part should be a Person, second part should be a Workplace". This is not captured at all in the schema that Dropshot generated above, but it is required by the actual endpoint implementation code I wrote.

But the OpenAPI spec supports using detailed schemas that explain each part's expected type. So in theory Dropshot could generate a more detailed schema that captures my expectations for each part.

As the person who originally added multipart support, this oversight is purely due to me being silly a few years ago, and I think this gap should be fixed.