Expose internal HttpClient

[esd-thg]

Is your feature request related to a problem? Please describe the problem.

Sometimes Kiota doesn't handle certain scenarios or generates incorrect "stuff" for an endpoint.
In these cases it would be handy to be able to access the HttpClient Kiota uses so we can manually make the request.
I'm expecting the HttpClient we get from Kiota to have the configured baseurl and authentication.

Client library/SDK language

Csharp

Describe the solution you'd like

public async Task GetAsync()
{
  // doesn't work - waiting on #7147
  // await _client.Tests.GetAsync(x => x.Headers.Add(HeaderNames.Accept, MediaTypeNames.Application.Octet));

  // workaround until fixed
  var httpClient = _client.HttpClient;
  // DefaultRequestHeaders might not be correct usage
  httpClient.DefaultRequestHeaders.Add(HeaderNames.Accept, MediaTypeNames.Application.Octet);
  var stream = await httpClient.GetStreamAsync("/tests");
  ...
}

Additional context

If DefaultRequestHeaders isn't the correct way to use the http client from Kiota then please make it easy to add headers to a single request.
Using HttpRequestMessage and SendAsync is tedious :)

[esd-thg]

meanwhile I found a rather cursed workaround

public async Task GetAsync()
{
    // doesn't work - waiting on #7147
    // await _client.Tests.GetAsync(x => x.Headers.Add(HeaderNames.Accept, MediaTypeNames.Application.Octet));

    // workaround until fixed
    var requestBuilder = _client.Tests;
    var requestInfo = requestBuilder.ToGetRequestInformation(x =>
            x.Headers.Add(HeaderNames.Accept, MediaTypeNames.Application.Octet)
        );
    var requestAdapter = GetRequestAdapter(requestBuilder);
    var stream = await requestAdapter.SendPrimitiveAsync<Stream>(requestInfo);
    ...
}

private static IRequestAdapter GetRequestAdapter(BaseRequestBuilder builder)
{
    // Access the protected RequestAdapter property via reflection
    var property = builder
        .GetType()
        .BaseType?.GetProperty(
            "RequestAdapter",
            System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance
        );

    return (IRequestAdapter)property?.GetValue(builder)!;
}

Please save us!