Support for multiple `requestBody` content types

In some schemas the requestBody has multiple content types. Here's an example from the github API:

        "operationId": "markdown/render-raw",
        "requestBody": {
          "content": {
            "text/plain": {
              "schema": {
                "type": "string"
              }
            },
            "text/x-markdown": {
              "schema": {
                "type": "string"
              }
            }
          }
        },

In that scenario we'd like to allow consumers to specify either content encoding. I'd suggest we do something like this:

pub enum MarkdownRenderRawBody {
    TextPlain(String),
    XMarkdown(String),
}

fn foo(client: &Client) {
    let _ = client.markdown_render_raw().body(MarkdownRenderRawBody::TextPlain("xyz".to_string())).send.await;
}

Start by tracing how the generator handles OpenAPI requestBody content schemas and how generated request builders expose bodies. Use the GitHub API example in the issue as a case to follow. Done means generated clients accept either listed content type through a typed body API, with coverage for multiple requestBody content types.