[Feature] autolearnstats: add --sort-by and --group options

[Feature] autolearnstats: add --sort-by and --group options

Add --sort-by <col> to sort rows by a chosen column (verdict, score,
ts, tid, ip, from, rcpts) with timestamp as a tiebreaker.  Score is
compared numerically; all other columns lexicographically.

Add --group flag to insert a blank separator line between consecutive
rows where the --sort-by key changes.

Add unit tests for sort key extraction functions.