Ziti_load_context does not process the pointer as json

[dovholuknf]

@ekoby informs me that Ziti_load_context is intended to process the pointer as either a file on disk or json. Using it for reading an identity file works, however using the pointer as json does not.

Please update the zitilib.h file and clarify that this pointer is either json or a file on disk. Also indicate the precedence (is it looking for a file first, then json, or json first then a file?). Also indicate if the path is absolute only or if it can be relative and finally, fix the function to take either a pointer to json or a pointer that represents a path on the file system