feat: extend C box API with a new auth method

1. Current commit introduces 'box_auth_data_prepare()' to prepare
   a data string for any supported authentication methods.
2. The user name argument is refactored in the auth methods: the
   null-terminated string is replaced with an address range approach.
   Now Rust users don't need to re-allocate username with CString.
3. Password length type was set to uint32_t (previously it was size_t,
   int, uint32_t for different functions). Tarantool uses murmur3a,
   so all the hashed strings should be up to 32 bit long.

NO_DOC=picodata internal patch
NO_CHANGELOG=picodata internal patch
NO_TEST=picodata internal patch