Add support for multiple trailing optional arguments

[nicdem-NA]

This PR introduces support for multiple trailing optional arguments :

• Extend optional argument support from a single trailing optional arg to multiple consecutive trailing optional args
• Update get_num_required_args() to walk backwards and count all trailing optional args, not just the last one
• Update process_line() to set default values for all omitted optional args, not just one
• Relax validate_arg_def() to no longer restrict optional args to only the last position

Example:
Previously, only the last argument of a command could be optional:

CONSOLE_COMMAND_DEF(cmd, "description",
CONSOLE_INT_ARG_DEF(required_arg, "A required arg"),
CONSOLE_OPTIONAL_INT_ARG_DEF(optional_arg, "An optional arg")  // only one allowed
); 

Now, multiple consecutive trailing arguments can be optional:

CONSOLE_COMMAND_DEF(cmd, "description",
CONSOLE_INT_ARG_DEF(required_arg, "A required arg"),
CONSOLE_OPTIONAL_INT_ARG_DEF(opt1, "First optional arg"), 
CONSOLE_OPTIONAL_INT_ARG_DEF(opt2, "Second optional arg")
);

Users can invoke this as cmd 1, cmd 1 2, or cmd 1 2 3. Omitted optional args receive CONSOLE_INT_ARG_DEFAULT or CONSOLE_STR_ARG_DEFAULT as before.