in VisionCamera v1 & v2 there were two ObjC macros that were helping
in creation/registration of Frame Processors, but these were removed with
v3
This PR reintroduces such macros, which will not only make FP development
easier, but also it will also fix issues people had with registration of
Swift Frame Processors (+load vs +initialize issues)
Docs were also updated to reflect that the macros should be used to
correctly initialize and register ObjC/Swift Frame Processors
