parse_target doesn't accept target specifier with back slash delineated Domain\Username format

[xchg-rax-rax]

parse_target in impacket/impacket/examples/utils.py is the function responsible for parsing the target specifier (eg. ARCHETYPE/sql_svc:M3g4c0rp123@10.129.203.99) from the command line arguments of the tools in examples.

The canonical format for a domain+user specifier used by windows is DOMAIN\Username with the domain and a username separated by a backslash.
The current implementation of parse_target only accepts specifiers of the form DOMAIN/Username, with a forward slash, and will misinterpret the domain to be part of the username in the event that a backslash is used instead.

The regular expression used to parse the command line argument should be modified to accept both formats.

This could be achieve by replacing the one currently used with the following:

(?:(?:([^\/\\@:]*)[\/\\])?([^@:]*)(?::([^@]*))?@)?(.*)