Feat. Request: HttpRequest/HttpResponse convenience methods.

Description

In order to implement the HttpClient interface in Java to embed a custom HttpClient in the SDK, there is the need to implement some ellaborate mapping for a few entities in the HttpRequest/HttpResponse.

1. Headers

Currently one can iterate through names and then get a List<String> for each. This results in code that looks like this:

HttpHeaders target = new HttpHeaders();
sourceHeaders.names().forEach(name -> {
    List<String> values = sourceHeaders.values(name);
    HttpHeaderName headerName = HttpHeaderName.fromString(name);
    if (values.isEmpty()) {
        target.set(headerName, "");
    } else {
        target.set(headerName, values);
    }
});
return target;

It would be nice to have something that returns the Headers as Map<String, String> or even Map<String, List<String>>

2. HttpResponseBody

Given the API, one has to "double read" the content of the body by calling fun writeTo(outputStream: OutputStream)

It would be nice to have an alternative that would return an java.io.InputStream so we can pipe the body directly to our type.

3. HttpRequestBody

Same as the above. Having the contents of the body returned as an InputStream would make things more ergonomic for us.

Thank you!

Locate the HttpHeaders, HttpRequestBody, and HttpResponseBody definitions, then inspect the existing names(), values(), and writeTo(OutputStream) APIs. Done means providing ergonomic header and InputStream access for request and response bodies while preserving the existing behavior; the issue does not name specific tests or files to run.