Implement Null ForwardOpen support

Null ForwardOpen support is not implemented yet.
For feature consistency with the V2 stack, this shall be done.

The connection path parsing seems to be not fully compliant with the specification and may be reworked with this ticket as well.

A config parameter shall allow the user to enable the feature on demand, default setting shall be off, i.e. not supported.

Null ForwardOpen shall be subject to ForwardOpen Forwarding with all features (i.e. allow the user to reject a particular request, allow attaching application response data for successful forward open).

If configuration data changes due to a NULL FwOpen the new configuration data shall be presented to the host application with a CL3_SERVICE_IND, as usual. The host shall have the option to reject or accept this new config data with arbitrary status code.

Testcases have to be implemented.

API documentation has to be extended.

Conformance Test CTT18 has to be considered.