Expose set_amxstring_utf8 to AMXX API

Considering function is a template, it was not possible to passe a function pointer ```REGISTER_FUNC```.
As solution and because function supports only 2 different types (char and cell), this patch introduces ```SetAmxStringUTF8Char``` and ```SetAmxStringUTF8Cell``` to API.
(Note: last commit, it's just copy-past of amxx files, just saying :P)