feat #1183  implementation of synchronous XHR requests

a new property has been added to aria.modules.RequestBeans.RequestObject and to aria.core.CfgBeans.IOAsyncRequestCfg, and the request method of aria.core.transport.BaseXHR class has been modified.
Close #1183
This bean is used also to document the request method for other transports. Maybe in the `$description` it is better to specify that it is only valid for the XHR transport