Add compareSubstrings function for string comparison in SQL

Add a new string comparison function, compareSubstrings to compare parts of two strings. Example: SELECT compareSubstrings('Saxony', 'Anglo-Saxon', 0, 6, 5) AS result means "compare 6 bytes of strings 'Saxon' and 'Anglo-Saxon' lexicographically, starting at offset 0 in the first string, offset 5 in the second string". #74070 (lgbo).