Add builtin functions for masking UTF-8 strings

The existing mask functions can only deal with ASCII characters. It will be very useful to provide mask functions that can deal with UTF-8 characters, or to improve current mask fuctions to deal with them as Hive does. Otherwise, Impala may leak information since we count each UTF-8 character as three. For example, if we want to mask the last two characters, Impala only masks the last UTF-8 character.

hive> select mask_last_n('SQL引擎', 2, 'x', 'x', 'x', 'x');
SQLxx
impala> select mask_last_n('SQL引擎', 2, 'x', 'x', 'x', 'x');
SQL引�xx

Some common scenarios:

• Masking the last two UTF-8 characters of Chinese names.
• Show only the first several UTF-8 characters of Chinese addresses and mask all the remaining characters.

However, this depends on our BE support for UTF-8 strings.