Add option to generate to_string functions for oneof messages

[pdietl]

What should the feature do?
Add an option like (nanopb_fileopt).enum_to_string, maybe called oneof_to_string that generates functions which take a pb_size_t and output a string displaying which oneof member is indicated.

In what situation would the feature be useful?
Debugging

[pdietl]

If someone thinks this is a good idea, I can implement it