make wolfSSL_UseALPN take const char* rather than char*

[ycdev1]

Currently wolfSSL_UseALPN takes char* argument which suggests that it mutates the data it receives while it just copies the data to a temporary allocated buffer so it should take const char* instead.

Also wolfSSL_ALPN_GetProtocol should output const char* but this may break existing code.

[wolfSSL-Bot]

Can one of the admins verify this patch?

[ycdev1]

Does wolfssl currently have an api to clear the set alpn? If no, the wolfSSL_UseALPN can clear the alpn if passed nullptr and 0