IFunctionBindingsFeature is internal and pevent unit testing in middleware

### Description

I'm currently writing a authentication middleware. When it identifies that the user bearer token is not valid it sets the response data to return a 401 response.

` public async Task Invoke(FunctionContext context, FunctionExecutionDelegate next)
{
HttpRequestData? request = await context.GetHttpRequestDataAsync();

if (request is null)
{
await next(context);

return;
}

try
{
var authService = context.InstanceServices.GetRequiredService();

AuthorizedContext authorizedContext = await authService.Authorize(request);

context.Items.Add("AuthorizedContext", authorizedContext);

await next(context);
}
catch (HttpRequestException exception)
{
LogErrorMessage(request, exception);
context.GetInvocationResult().Value = request.CreateErrorResponse(exception);
}
catch (Exception exception)
{
LogErrorMessage(request, exception);
context.GetInvocationResult().Value = request.CreateErrorResponse(exception);
}
`

The function `GetInvocationResult` on context is a static function that calls another static function `GetBindings`. `GetBindings` executes this line of code: `context.Features.GetRequired();`. `GetRequired` is also a static function that calls `features.Get`

I would need to to create unit tests but I cannot mock static functions. I'd have to go mocking `features.Get` but it is impossible because IFunctionBindingsFeature is an internal interface.

I've been wondering if it is possible to open this up to allow unit testing possible. I want to be able to add unit tests when a user cannot be authenticated and check that the middleware returns the correct error.