ADD access to request config

The HttpClientFactory creates an Apache RequestConfig (DefaultRequestConfig) from the OSGi config and passes it as default to the Apache HttpClientBuilder. When Apache HttpClient is returned from factory, there exists no access to the DefaultRequestConfig.

It is possible to add a custom RequestConfig to the Apache HttpRequestBase. But for not defined parameters, it will override the values from the DefaultRequestConfig by default Apache values - hence the settings defined in the OSGi config get lost.

Having access to the DefaultRequestConfig, allows to make a copy with custom values by using RequestConfig#copy method.

Please extend the HttpClientFactory by giving access to a RequestConfig for an url.