UJOTYP-45: The serialization should handle UTF8-string variant type

• modify "ujot_write_string_to_ujo" to handle C-strings and UTF8-strings
• get rid of "size_t" for string/buffer sizes (change to "ujotUInt32")
• extend "ujot_on_parsed_ujo_element" for string sub type UTF8
• change type of UTF8 strings to "unsigned char *"
• add test helper functions for lists and maps to use UTF8 strings
• add UTF8 strings to test structure
• refactor of test structure verification function
• add test helper function to compare UTF8 strings
• modify UTF8 string variant test to use new UTF8 compare function