[CustomHandler] Protect against external HTTP requests accessing handler endpoints

With HttpRequestForward enabled and this configuration:

* `MyHttpTrigger` function that has a catch-all route that forwards all requests to the handler
* `MyQueueTrigger` function

It's possible for an external HTTP call to `/MyQueueTrigger` to be routed to the handler endpoint. This is unexpected and potentially a security concern.

Some possible solutions:

* Update runtime to block HTTP requests to handler endpoints
* Help the developer protect against this by passing a header in all HTTP requests to indicate a call originated from external

My preference is to automatically blocking those requests.

@pragnagopa @yojagad