Add --backslash option to clickhouse-format for line-ending backslashes

Why it matters

This feature helps users produce formatted SQL queries with line continuation characters (backslashes) at the end of each line, facilitating multi-line query handling or copy-pasting query snippets into scripts or interactive environments where backslash continuation is needed.

How to use it

Run clickhouse-format with the --backslash option. For example:

clickhouse-format --backslash < input.sql

This will output the formatted query with a backslash appended at the end of every line.