Provide API calls for functions and values in rig caps and state structures

[mdblack98]

WSJT-X and others have had to resort to using rig_caps to get vfo_list, function availability and others to get needed info from hamlib.
Need to implement API calls in 4.X and eventually make these structures private in 5.0

This will avoid shared library problems when structures change.

Support read and set rit
if (myRig->caps->get_rit == nullptr)
if (myRig->caps->set_rit == nullptr)

Whether the radio can set/get vfo
if (myRig->caps->get_vfo == nullptr)
if (myRig->caps->set_vfo == nullptr)

[mdblack98]

Adding
void *rig_get_function_ptr(rig_model_t, enum rig_function_e rig_function);
e.g.
void *p = rig_get_function(rig_model, RIG_FUNCTION_SET_VFO);
same as rig->caps->set_vfo